{"id":34547,"date":"2026-03-17T11:11:54","date_gmt":"2026-03-17T11:11:54","guid":{"rendered":"https:\/\/1grid.co.za\/knowledge\/?post_type=ht_kb&#038;p=34547"},"modified":"2026-03-17T11:30:43","modified_gmt":"2026-03-17T11:30:43","slug":"smtp-error-421-temporary-server-unavailable-how-to-fix","status":"publish","type":"ht_kb","link":"https:\/\/1grid.co.za\/knowledge\/smtp-error-421-temporary-server-unavailable-how-to-fix-it\/","title":{"rendered":"SMTP Error 421: Temporary Server Unavailable (How to Fix It)"},"content":{"rendered":"\n<p>If&nbsp;you\u2019ve&nbsp;received&nbsp;<strong>SMTP Error 421<\/strong>, it means the receiving mail server is temporarily unavailable or too busy to process your email.&nbsp;<\/p>\n\n\n\n<p>This guide will:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-explain-what-smtp-421-means\">Explain what SMTP 421 means&nbsp;<\/li>\n\n\n\n<li id=\"htoc-help-you-identify-why-the-message-was-deferred\">Help you&nbsp;identify&nbsp;why the message was deferred&nbsp;<\/li>\n\n\n\n<li id=\"htoc-show-you-how-to-safely-resend-or-adjust-sending-practices\">Show you how to safely resend or adjust sending practices&nbsp;<\/li>\n\n\n\n<li id=\"htoc-clarify-that-the-issue-is-temporary-and-usually-not-related-to-your-hosting\">Clarify that the issue is temporary and usually not related to your hosting&nbsp;<\/li>\n<\/ul>\n\n\n\n<p>By the end of this guide,&nbsp;you\u2019ll&nbsp;confidently manage 421 errors without unnecessary support escalation.&nbsp;<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"aligncenter size-full\"><img loading=\"lazy\" decoding=\"async\" width=\"1600\" height=\"600\" src=\"https:\/\/1grid.co.za\/knowledge\/wp-content\/uploads\/2026\/03\/1-grid-Images-for-KB-Articles-1000-x-600-px-9.png\" alt=\"SMTP Error 421: Temporary Server Unavailable (How to Fix It) \" class=\"wp-image-34548\"\/><\/figure>\n<\/div>\n\n\n<div class=\"wp-block-ht-block-toc is-style-rounded htoc htoc--position-wide toc-list-style-plain\" data-htoc-state=\"expanded\"><span class=\"htoc__title\"><span class=\"ht_toc_title\">Table of Contents<\/span><span class=\"htoc__toggle\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"16\" height=\"16\"><g fill=\"#444\"><path d=\"M15 7H1c-.6 0-1 .4-1 1s.4 1 1 1h14c.6 0 1-.4 1-1s-.4-1-1-1z\"><\/path><path d=\"M15 1H1c-.6 0-1 .4-1 1s.4 1 1 1h14c.6 0 1-.4 1-1s-.4-1-1-1zM15 13H1c-.6 0-1 .4-1 1s.4 1 1 1h14c.6 0 1-.4 1-1s-.4-1-1-1z\"><\/path><\/g><\/svg><\/span><\/span><div class=\"htoc__itemswrap\"><ul class=\"ht_toc_list\"><li class=\"\"><a href=\"#htoc-what-smtp-error-421-means\">What SMTP Error&nbsp;421&nbsp;Means&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-why-this-matters\">Why This&nbsp;Matters&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-step-by-step-fix-for-smtp-421-error-temporary-server-unavailable\">Step-by-Step&nbsp;Fix&nbsp;for&nbsp;SMTP 421&nbsp;Error&nbsp;\u2013 Temporary Server Unavailable&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-quick-fixes-for-common-smtp-552-variations\">Quick&nbsp;Fixes for Common SMTP 552&nbsp;Variations&nbsp;&nbsp;&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-advanced-optional-fixes-for-technical-users\">Advanced&nbsp;(Optional)&nbsp;Fixes&nbsp;(For Technical Users)&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-before-you-move-on\">Before You Move&nbsp;On&nbsp;&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-advanced-optional-fixes-for-technical-users1\">Additional Advanced Checks<\/a><\/li><li class=\"\"><a href=\"#htoc-if-this-didn-t-work\">If This&nbsp;Didn\u2019t&nbsp;Work&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-common-mistakes-to-avoid\">Common Mistakes to&nbsp;Avoid&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-empowering-insight\">Empowering Insight&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-next-recommended-steps\">Next Recommended Steps&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-faqs\">FAQs&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-additional-resources\">Additional&nbsp;Resources&nbsp;<\/a><\/li><\/ul><\/div><\/div>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-what-smtp-error-421-means\"><strong>What SMTP Error&nbsp;421&nbsp;Means<\/strong>&nbsp;<\/h2>\n\n\n\n<p>SMTP 421 is a&nbsp;<strong>temporary rejection (soft bounce)<\/strong>.&nbsp;<\/p>\n\n\n\n<p>Common scenarios include:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-the-recipient-server-is-overloaded-or-busy\">The recipient server is overloaded or busy&nbsp;<\/li>\n\n\n\n<li id=\"htoc-greylisting-is-enabled\">Greylisting&nbsp;is enabled&nbsp;<\/li>\n\n\n\n<li id=\"htoc-connection-or-port-limits-have-been-reached\">Connection or port limits have been reached&nbsp;<\/li>\n\n\n\n<li id=\"htoc-downstream-servers-are-temporarily-unavailable\">Downstream servers are temporarily unavailable&nbsp;<\/li>\n<\/ul>\n\n\n\n<p>Unlike <strong><a href=\"https:\/\/1grid.co.za\/knowledge\/why-emails-bounce-back-error-messages-and-how-to-fix-them\/#htoc-hard-bounce-permanent-failure\" target=\"_blank\" rel=\"noreferrer noopener\">hard bounces<\/a><\/strong> (5xx codes),\u00a0<strong>421 messages are typically retried automatically<\/strong>\u00a0by your mail server as they are <strong><a href=\"https:\/\/1grid.co.za\/knowledge\/why-emails-bounce-back-error-messages-and-how-to-fix-them\/#htoc-soft-bounce-temporary-failure\" target=\"_blank\" rel=\"noreferrer noopener\">temporary<\/a> <\/strong>failures.\u00a0<\/p>\n\n\n\n<p><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/a-guide-to-smtperror-codes\/\" target=\"_blank\" rel=\"noreferrer noopener\">Extended codes<\/a> <\/strong>(e.g., 4.3.2, 4.7.1)\u00a0indicate\u00a0the specific temporary cause.\u00a0<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-why-this-matters\"><strong>Why This&nbsp;Matters<\/strong>&nbsp;<\/h2>\n\n\n\n<p>Ignoring 421 errors can:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-delay-email-delivery-to-clients-or-customers\">Delay email delivery to clients or customers&nbsp;<\/li>\n\n\n\n<li id=\"htoc-trigger-repeated-soft-bounces-if-high-volume-sending-persists\">Trigger repeated soft bounces if high-volume sending persists&nbsp;<\/li>\n\n\n\n<li id=\"htoc-cause-confusion-over-whether-your-messages-are-being-lost\">Cause confusion over whether your messages are being lost&nbsp;<\/li>\n\n\n\n<li id=\"htoc-lead-to-unnecessary-support-tickets\">Lead to unnecessary support tickets&nbsp;<\/li>\n<\/ul>\n\n\n\n<p>Understanding the root cause ensures you manage retries safely and avoid overloading your server.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-step-by-step-fix-for-smtp-421-error-temporary-server-unavailable\"><strong>Step-by-Step&nbsp;Fix&nbsp;for&nbsp;SMTP 421&nbsp;Error&nbsp;\u2013 Temporary Server Unavailable<\/strong>&nbsp;<\/h2>\n\n\n\n<p>Follow these steps in order.&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>1: Confirm the Full Error Message&nbsp;<\/strong><\/summary>\n<p id=\"htoc-1-confirm-the-full-error-message\">Open the bounce message or delivery notification.&nbsp;<\/p>\n\n\n\n<p id=\"htoc-ensure-it-specifically-references-variations-of\">Ensure it specifically references variations of:&nbsp;<\/p>\n\n\n\n<pre id=\"htoc-421-service-not-available\" class=\"wp-block-code\"><code>421 Service not available&nbsp;\n421 Temporary&nbsp;server&nbsp;unavailable&nbsp;\n421 4.x.x Temporary error&nbsp;\n421 Server busy, try again later&nbsp;\n421 Too many connections&nbsp;\n421&nbsp;Greylisting&nbsp;enabled&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-1-confirm-the-full-error-message1\">If the error shows 550, 552, 554, or 535 instead, refer to our <strong><a href=\"https:\/\/1grid.co.za\/knowledge\/a-guide-to-smtperror-codes\/\" target=\"_blank\" rel=\"noreferrer noopener\">SMTP Error Codes<\/a> <\/strong>guide.\u00a0<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>2: Wait Before Retrying&nbsp;<\/strong><\/summary>\n<p id=\"htoc-2-wait-before-retrying\">SMTP 421 is a\u00a0<strong>temporary (<a href=\"https:\/\/1grid.co.za\/knowledge\/why-emails-bounce-back-error-messages-and-how-to-fix-them\/#htoc-soft-bounce-temporary-failure\" target=\"_blank\" rel=\"noreferrer noopener\">soft bounce<\/a>) error<\/strong>.\u00a0<\/p>\n\n\n\n<p id=\"htoc-in-most-cases-it-resolves-automatically-within\">In most cases, it resolves automatically within:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-15-to-30-minutes\">15 to 30 minutes&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-do-not-repeatedly-click-send-instead\">Do not repeatedly click \u201cSend.\u201d Instead:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-wait-at-least-15-minutes\">Wait at least 15 minutes&nbsp;<\/li>\n\n\n\n<li id=\"htoc-retry-once\">Retry once&nbsp;<\/li>\n\n\n\n<li id=\"htoc-allow-automatic-server-retry-attempts-to-complete\">Allow automatic server retry attempts to complete&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-2-wait-before-retrying1\">Repeated sending can increase temporary connection limits.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>3: Reduce Sending Volume (If Applicable)&nbsp;<\/strong><\/summary>\n<p id=\"htoc-3-reduce-sending-volume-if-applicable\">If you are sending multiple emails or bulk mail:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-reduce-simultaneous-smtp-connections\">Reduce simultaneous SMTP connections&nbsp;<\/li>\n\n\n\n<li id=\"htoc-send-in-smaller-batches\">Send in smaller batches&nbsp;<\/li>\n\n\n\n<li id=\"htoc-add-delays-between-sending-groups\">Add delays between sending groups&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-3-reduce-sending-volume-if-applicable1\"><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/email-restrictions\/#htoc-1-emails-per-hour-sending-volume-limit\" target=\"_blank\" rel=\"noreferrer noopener\">High-volume sending<\/a> <\/strong>is one of the most common triggers for 421 errors.\u00a0<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>4: Check Recipient Server Status&nbsp;<\/strong><\/summary>\n<p id=\"htoc-step-4-check-recipient-server-status\">Confirm whether the recipient\u2019s mail server is:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-under-maintenance\">Under maintenance&nbsp;<\/li>\n\n\n\n<li id=\"htoc-experiencing-temporary-downtime\">Experiencing temporary downtime&nbsp;<\/li>\n\n\n\n<li id=\"htoc-applying-greylisting-policies\">Applying&nbsp;greylisting&nbsp;policies&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-major-providers-gmail-yahoo-outlook-aol-may-temporarily-defer-connections-during-peak-load\">Major providers (Gmail, Yahoo, Outlook, AOL) may temporarily defer connections during peak load.&nbsp;<\/p>\n\n\n\n<p id=\"htoc-step-4-check-recipient-server-status1\">If the issue affects only one&nbsp;recipient&#8217;s&nbsp;domain, the cause is likely on their side.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>5: Send a Simple Test Email&nbsp;<\/strong><\/summary>\n<p id=\"htoc-step-5-send-a-simple-test-email\">Try sending:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-a-plain-text-email\">A plain-text email&nbsp;<\/li>\n\n\n\n<li id=\"htoc-no-attachments\">No attachments&nbsp;<\/li>\n\n\n\n<li id=\"htoc-no-images\">No images&nbsp;<\/li>\n\n\n\n<li id=\"htoc-one-recipient-only\">One recipient only&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-step-5-send-a-simple-test-email1\">If this succeeds, the original message may have triggered temporary rate or filtering <a href=\"https:\/\/1grid.co.za\/knowledge\/email-restrictions\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>sending limits<\/strong><\/a>.\u00a0<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>6: Review Delivery Logs (Basic&nbsp;Level)&nbsp;<\/strong><\/summary>\n<p id=\"htoc-step-6-review-delivery-logs-basic-level\">Log in to:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-cpanel-email-track-delivery\"><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/how-to-log-in-to-cpanel-via-your-1-grid-customer-zone\/\" target=\"_blank\" rel=\"noreferrer noopener\">cPanel<\/a> <\/strong>\u2192 Email \u2192 <strong><a href=\"https:\/\/1grid.co.za\/knowledge\/cpanel-how-to-view-delivery-reports-via-track-delivery\/\" target=\"_blank\" rel=\"noreferrer noopener\">Track Delivery<\/a>\u00a0<\/strong><\/li>\n\n\n\n<li id=\"htoc-plesk-websites-amp-domains-select-your-domain-mail-track-email-delivery\"><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/plesk-how-do-i-login-via-customer-zone\/\" target=\"_blank\" rel=\"noreferrer noopener\">Plesk<\/a> <\/strong>\u2192\u00a0Websites &amp; Domains \u2192 Select your domain \u2192 Mail \u2192 Track Email Delivery\u00a0<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-check-the-message-status\">Check the message status:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-deferred-temporary-delay-normal-for-421\">Deferred = Temporary delay (normal for 421)&nbsp;<\/li>\n\n\n\n<li id=\"htoc-failed-rejected-different-issue-refer-to-specific-smtp-guide\">Failed\/Rejected = Different issue (refer to specific SMTP guide)&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-step-6-review-delivery-logs-basic-level1\">421 messages are typically queued and retried automatically.&nbsp;<\/p>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-quick-fixes-for-common-smtp-552-variations\"><strong>Quick&nbsp;Fixes for Common SMTP 552&nbsp;Variations&nbsp;&nbsp;<\/strong>&nbsp;<\/h2>\n\n\n\n<p>SMTP 421 occurs when:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-the-recipient-server-is-temporarily-unavailable\">The recipient server is temporarily unavailable&nbsp;<\/li>\n\n\n\n<li id=\"htoc-server-connection-limits-or-greylisting-are-applied\">Server connection limits or&nbsp;greylisting&nbsp;are applied&nbsp;<\/li>\n\n\n\n<li id=\"htoc-temporary-routing-or-downstream-server-errors-occur\">Temporary routing or downstream server errors occur&nbsp;<\/li>\n<\/ul>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\"><\/ol>\n\n\n\n<ol start=\"2\" class=\"wp-block-list\"><\/ol>\n\n\n\n<ol start=\"3\" class=\"wp-block-list\"><\/ol>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>1.&nbsp;421 4.2.1 \/ 4.3.x: Temporary Resource Limits&nbsp;<\/strong><\/summary>\n<p id=\"htoc-1-421-4-2-1-4-3-x-temporary-resource-limits\">Example:&nbsp;<\/p>\n\n\n\n<pre id=\"htoc-421-4-2-1-dyn-t1-https-postmaster-info-aol-com-errors-421dynt1-html-421-4-3-1-out-of-disk-space-for-temp-file-421-4-3-2-all-server-ports-are-busy\" class=\"wp-block-code\"><code>421&nbsp;4.2.1 :&nbsp;(DYN:T1) https:\/\/postmaster.info.aol.com\/errors\/421dynt1.html&nbsp;\n421 4.3.1 Out of disk space for temp file&nbsp;\n421 4.3.2 All server ports are busy&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-what-it-means-the-recipient-server-cannot-accept-messages-right-now-due-to-temporary-resource-limitations-disk-space-ports-memory\"><strong>What It Means:<\/strong>&nbsp;<br>The recipient server cannot accept messages right now due to temporary resource limitations (disk space, ports, memory).&nbsp;<\/p>\n\n\n\n<p id=\"htoc-how-to-fix-it\"><strong>How to Fix It:<\/strong>&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-wait-15-to-30-minutes-before-retrying-the-email\">Wait 15&nbsp;to&nbsp;30 minutes before retrying the email&nbsp;<\/li>\n\n\n\n<li id=\"htoc-reduce-the-number-of-emails-sent-simultaneously-if-sending-in-bulk\">Reduce the number of emails sent simultaneously if sending in bulk&nbsp;<\/li>\n\n\n\n<li id=\"htoc-for-high-volume-senders-stagger-or-batch-messages\">For high-volume senders,&nbsp;stagger&nbsp;or batch messages&nbsp;<\/li>\n\n\n\n<li id=\"htoc-check-recipient-server-status-if-available\">Check the recipient server status if available&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-1-421-4-2-1-4-3-x-temporary-resource-limits1\"><strong>Reassurance:<\/strong>&nbsp;This is temporary and does&nbsp;<strong>not affect your hosting or mailbox<\/strong>.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>2.&nbsp;421 4.4.2 \/ 4.4.5: Timeout or Connection Limits&nbsp;<\/strong><\/summary>\n<p id=\"htoc-2-421-4-4-2-4-4-5-timeout-or-connection-limits\">Example:&nbsp;<\/p>\n\n\n\n<pre id=\"htoc-421-4-4-2-mx-aol-com-error-timeout-exceeded-421-4-4-5-too-many-connections-from-your-host\" class=\"wp-block-code\"><code>421 4.4.2 mx.aol.com Error: timeout exceeded&nbsp;\n421 #4.4.5 Too many connections from your host&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-what-it-means-your-mail-server-attempted-to-connect-but-the-recipient-server-timed-out-or-exceeded-concurrent-connection-limits\"><strong>What It Means:<\/strong>&nbsp;<br>Your mail server&nbsp;attempted&nbsp;to connect, but the recipient server timed out or exceeded concurrent connection limits.&nbsp;<\/p>\n\n\n\n<p id=\"htoc-how-to-fix-it1\"><strong>How to Fix It:<\/strong>&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-retry-sending-after-a-short-delay\">Retry sending after a short delay&nbsp;<\/li>\n\n\n\n<li id=\"htoc-reduce-simultaneous-smtp-connections1\">Reduce simultaneous SMTP connections&nbsp;<\/li>\n\n\n\n<li id=\"htoc-for-administrators-review-mail-queue-and-delivery-logs\">For administrators: review mail queue and delivery logs&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-2-421-4-4-2-4-4-5-timeout-or-connection-limits1\"><strong>Reassurance:<\/strong>&nbsp;The message is&nbsp;<strong>queued or deferred<\/strong>, not lost.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>3.&nbsp;421 4.7.x \/&nbsp;Greylisting&nbsp;\/ Policy-Based Temporary Rejection&nbsp;<\/strong><\/summary>\n<p id=\"htoc-3-421-4-7-x-greylisting-policy-based-temporary-rejection\">Example:&nbsp;<\/p>\n\n\n\n<pre id=\"htoc-421-4-7-0-mx-aol-com-error-too-many-errors-421-4-7-1-pol-t2-connection-refused-421-greylisting-enabled-please-try-later\" class=\"wp-block-code\"><code>421 4.7.0 mx.aol.com Error: too many errors&nbsp;\n421 4.7.1 POL-T2 Connection refused&nbsp;\n421&nbsp;Greylisting&nbsp;enabled, please try later&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-what-it-means-the-recipient-server-is-using-greylisting-or-temporary-policy-rules-to-defer-your-message-often-as-an-anti-spam-measure\"><strong>What It Means:<\/strong>&nbsp;<br>The recipient server is using&nbsp;greylisting&nbsp;or temporary policy rules to defer your message, often as an anti-spam measure.&nbsp;<\/p>\n\n\n\n<p id=\"htoc-how-to-fix-it11\"><strong>How to Fix It:<\/strong>&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-wait-the-recommended-retry-interval-usually-15-to-60-minutes\">Wait for the recommended retry interval (usually 15&nbsp;to&nbsp;60 minutes)&nbsp;<\/li>\n\n\n\n<li id=\"htoc-ensure-your-sending-server-follows-proper-spf-dkim-policies\">Ensure your sending server follows proper SPF\/DKIM policies&nbsp;<\/li>\n\n\n\n<li id=\"htoc-avoid-sending-large-bursts-of-emails-in-quick-succession\">Avoid sending large bursts of emails in quick succession&nbsp;<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-contact-recipient-postmaster-if-persistent-delays-occur\">Contact the recipient postmaster if persistent delays occur&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-3-421-4-7-x-greylisting-policy-based-temporary-rejection1\"><strong>Reassurance:<\/strong>&nbsp;These delays are&nbsp;<strong>normal for temporary&nbsp;policies<\/strong>,&nbsp;and your server or domain is not blocked.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>4. 421 Downstream Server Errors \/ Unexpected Failures&nbsp;<\/strong><\/summary>\n<p id=\"htoc-4-421-downstream-server-errors-unexpected-failures\">Example:&nbsp;<\/p>\n\n\n\n<pre id=\"htoc-421-downstream-server-error-421-fork-failed-421-unexpected-failure-please-try-later\" class=\"wp-block-code\"><code>421 Downstream server error&nbsp;\n421 Fork failed&nbsp;\n421 Unexpected&nbsp;failure, please try later&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-what-it-means-a-temporary-issue-occurred-on-a-downstream-server-between-you-and-the-recipient\"><strong>What It Means:<\/strong>&nbsp;<br>A temporary issue occurred on a downstream server between you and the recipient.&nbsp;<\/p>\n\n\n\n<p id=\"htoc-how-to-fix-it111\"><strong>How to Fix It:<\/strong>&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-retry-sending-after-waiting\">Retry sending after waiting&nbsp;<\/li>\n\n\n\n<li id=\"htoc-check-delivery-logs-to-ensure-the-email-is-deferred-not-rejected\">Check delivery logs to ensure the email is deferred, not rejected&nbsp;<\/li>\n\n\n\n<li id=\"htoc-if-persistent-contact-the-recipient-s-mail-provider-for-status\">If persistent, contact the recipient\u2019s mail provider for status&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-4-421-downstream-server-errors-unexpected-failures1\"><strong>Reassurance:<\/strong>&nbsp;These errors are temporary and&nbsp;<strong>do not&nbsp;indicate&nbsp;a problem with your hosting or email configuration<\/strong>.<\/p>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-advanced-optional-fixes-for-technical-users\"><strong>Advanced&nbsp;(Optional)&nbsp;Fixes&nbsp;(For Technical Users)<\/strong>&nbsp;<\/h2>\n\n\n\n<p>Administrators, Developers, or Technical Users can&nbsp;identify&nbsp;server-level causes, monitor retries and apply advanced configuration adjustments.&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>1. Check Mail Logs&nbsp;<\/strong><\/summary>\n<p id=\"htoc-1-check-mail-logs\">SMTP 421 errors are often temporary, but repeated occurrences may&nbsp;indicate&nbsp;server-level issues. Review logs to pinpoint patterns:&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Exim (default on cPanel\/WHM)&nbsp;<\/strong><\/summary>\n<pre id=\"htoc-exim-default-on-cpanel-whm\" class=\"wp-block-code\"><code>grep \"421\" \/var\/log\/exim_mainlog&nbsp;\ntail -f \/var\/log\/exim_mainlog&nbsp;<\/code><\/pre>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Postfix&nbsp;<\/strong><\/summary>\n<pre id=\"htoc-postfix\" class=\"wp-block-code\"><code>grep \"421\" \/var\/log\/maillog&nbsp;\ntail -f \/var\/log\/maillog&nbsp;<\/code><\/pre>\n<\/details>\n\n\n\n<p id=\"htoc-key-insights\"><strong>Key Insights:<\/strong>&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Correlate timestamps with high load periods&nbsp;<\/li>\n\n\n\n<li>Identify&nbsp;the recipient domains&nbsp;frequently&nbsp;returning 421&nbsp;<\/li>\n\n\n\n<li>Determine&nbsp;if errors are temporary resource issues,&nbsp;greylisting, or connection limits&nbsp;<\/li>\n<\/ul>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>2. Review Mail Queue&nbsp;<\/strong><\/summary>\n<p id=\"htoc-2-review-mail-queue\">Deferred messages due to 421 can accumulate. Check the queue to verify delivery status:&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Exim&nbsp;<\/strong><\/summary>\n<pre id=\"htoc-exim\" class=\"wp-block-code\"><code>exim&nbsp;-bp&nbsp;\nexim -Mrm&nbsp;&lt;message-id&gt;&nbsp;&nbsp; # Remove stuck messages if needed<\/code><\/pre>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Postfix&nbsp;<\/strong><\/summary>\n<pre id=\"htoc-postfix1\" class=\"wp-block-code\"><code>postqueue&nbsp;-p&nbsp;\npostsuper -d &lt;queue-id&gt;&nbsp; #&nbsp;Delete&nbsp;stuck message if necessary&nbsp;<\/code><\/pre>\n<\/details>\n\n\n\n<p id=\"htoc-notes\"><strong>Notes:<\/strong>&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Observe retry intervals and backoff behaviour&nbsp;<\/li>\n\n\n\n<li>Avoid mass deletion unless you confirm messages are truly undeliverable&nbsp;<\/li>\n<\/ul>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>3. Monitor Connections &amp; Limits&nbsp;<\/strong><\/summary>\n<p id=\"htoc-3-monitor-connections-amp-limits\">421 errors often occur due to connection throttling:&nbsp;<\/p>\n\n\n\n<p id=\"htoc-check-current-smtp-connections\"><strong>Check current SMTP connections:<\/strong>&nbsp;<\/p>\n\n\n\n<pre id=\"htoc-netstat-anp-grep-25-lsof-i-25\" class=\"wp-block-code\"><code>netstat -anp&nbsp;| grep :25&nbsp;<br>lsof -i&nbsp;:25&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-adjust-server-limits-for-high-volume-senders\"><strong>Adjust server limits (for high-volume senders):<\/strong>&nbsp;<\/p>\n\n\n\n<pre id=\"htoc-exim-etc-exim-exim-conf-increase-smtp-accept-max-or-smtp-accept-max-per-host\" class=\"wp-block-code\"><code>Exim:&nbsp;\/etc\/exim\/exim.conf&nbsp;\u2013 increase&nbsp;smtp_accept_max&nbsp;or&nbsp;smtp_accept_max_per_host&nbsp;<\/code><\/pre>\n\n\n\n<pre id=\"htoc-postfix-etc-postfix-main-cf-adjust-default-process-limit-and-smtpd-client-connection-count-limit\" class=\"wp-block-code\"><code>Postfix:&nbsp;\/etc\/postfix\/main.cf&nbsp;\u2013 adjust&nbsp;default_process_limit&nbsp;and&nbsp;smtpd_client_connection_count_limit&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-3-monitor-connections-amp-limits1\"><strong>Caution:<\/strong>&nbsp;Changes affect global mail throughout, so always test incrementally.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>4.&nbsp;Greylisting&nbsp;\/ Temporary Policies&nbsp;<\/strong><\/summary>\n<p id=\"htoc-4-greylisting-temporary-policies\">Some 421 responses come from recipient servers\u2019 anti-spam&nbsp;greylisting:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-temporarily-defer-messages-and-retry-after-15-60-minutes\">Temporarily defer messages and retry after 15 to 60 minutes&nbsp;<\/li>\n\n\n\n<li id=\"htoc-check-spf-dkim-alignment-and-ensure-your-ip-has-a-consistent-sending-identity\">Check SPF\/DKIM alignment and ensure your IP has a consistent sending identity&nbsp;<\/li>\n\n\n\n<li id=\"htoc-use-tools-like-swaks-or-telnet-to-simulate-smtp-sending-and-verify-responses\">Use tools like&nbsp;swaks&nbsp;or&nbsp;telnet&nbsp;to simulate SMTP sending and verify responses:&nbsp;<\/li>\n<\/ul>\n\n\n\n<pre id=\"htoc-swaks-to-recipient-example-com-server-mx-example-com-data-subject-test-n-nhello\" class=\"wp-block-code\"><code>swaks&nbsp;--to&nbsp;<a href=\"mailto:recipient@example.com\" target=\"_blank\" rel=\"noreferrer noopener\">recipient@example.com<\/a>&nbsp;--server mx.example.com --data \"Subject: Test\\n\\nHello\"&nbsp;<\/code><\/pre>\n\n\n\n<pre id=\"htoc-telnet-mx-example-com-25\" class=\"wp-block-code\"><code>telnet mx.example.com 25&nbsp;<\/code><\/pre>\n\n\n\n<p id=\"htoc-4-greylisting-temporary-policies1\"><strong>Goal:<\/strong>&nbsp;Confirm retries are&nbsp;accepted,&nbsp;and&nbsp;greylisting&nbsp;policies are obeyed.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>5. Network &amp; DNS Checks&nbsp;<\/strong><\/summary>\n<p id=\"htoc-5-network-amp-dns-checks\">421 errors can sometimes be linked to routing issues or DNS resolution failures:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-verify-mx-records\">Verify MX records:&nbsp;<\/li>\n<\/ul>\n\n\n\n<pre id=\"htoc-dig-mx-example-com\" class=\"wp-block-code\"><code>dig MX example.com&nbsp;<\/code><\/pre>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-test-network-reachability\">Test network reachability:&nbsp;<\/li>\n<\/ul>\n\n\n\n<pre id=\"htoc-ping-mx-example-com-traceroute-mx-example-com\" class=\"wp-block-code\"><code>ping mx.example.com&nbsp;<br>traceroute mx.example.com&nbsp;<\/code><\/pre>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Confirm reverse DNS (PTR) records are correctly set&nbsp;<\/li>\n<\/ul>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>6. Automation &amp; Retry Policies&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>Configure your MTA (Exim\/Postfix) with sensible retry intervals:&nbsp;<\/li>\n\n\n\n<li>Short delays (15&nbsp;to&nbsp;30 minutes) for transient errors&nbsp;<\/li>\n\n\n\n<li>Longer exponential backoff for persistent failures&nbsp;<\/li>\n\n\n\n<li>Ensure queue processing is not blocked by&nbsp;firewall&nbsp;rules or CPU\/memory constraints&nbsp;<\/li>\n<\/ul>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-before-you-move-on\"><strong>Before You Move&nbsp;On<\/strong><strong>&nbsp;<\/strong>&nbsp;<\/h2>\n\n\n\n<p id=\"htoc-\">Make sure you have done the following:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-confirm-your-message-size-and-format-are-within-limits\">Confirmed your message size and format are within limits&nbsp;<\/li>\n\n\n\n<li id=\"htoc-check-recipient-server-status\">Checked the recipient server status&nbsp;<\/li>\n\n\n\n<li id=\"htoc-reduce-simultaneous-sending-if-needed\">Reduced simultaneous sending, if needed&nbsp;<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-advanced-optional-fixes-for-technical-users1\"><strong>Additional Advanced Checks<\/strong><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-verify-mail-logs-show-successful-retries-after-previous-421-errors\">Verify mail logs show successful retries after&nbsp;previous&nbsp;421 errors&nbsp;<\/li>\n\n\n\n<li id=\"htoc-confirm-no-stuck-queue-messages-remain\">Confirm no stuck queue messages&nbsp;remain&nbsp;<\/li>\n\n\n\n<li id=\"htoc-check-that-connection-and-resource-limits-are-within-safe-thresholds\">Check that connection and resource limits are within safe thresholds&nbsp;<\/li>\n\n\n\n<li id=\"htoc-ensure-spf-dkim-and-ptr-alignment-to-avoid-future-temporary-rejections\">Ensure SPF, DKIM, and PTR alignment to avoid future temporary rejections&nbsp;<\/li>\n<\/ul>\n\n\n\n<p><strong>Reassurance:&nbsp;<\/strong>These advanced steps are&nbsp;<strong>optional for administrators<\/strong>. Standard users should rely on the beginner-level guidance: 421 is a&nbsp;<strong>temporary, recoverable issue<\/strong>&nbsp;and does&nbsp;<strong>not affect hosting, domain registration, or stored mailboxes<\/strong>.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-if-this-didn-t-work\"><strong>If This&nbsp;Didn\u2019t&nbsp;Work<\/strong>&nbsp;<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-investigate-server-logs-for-persistent-deferred-messages\">Investigate server logs for persistent deferred messages&nbsp;<\/li>\n\n\n\n<li id=\"htoc-contact-recipient-postmaster-if-necessary\">Contact the recipient postmaster if necessary&nbsp;<\/li>\n\n\n\n<li id=\"htoc-confirm-batch-size-smtp-limits-and-greylisting-policies\">Confirm batch size, SMTP limits, and&nbsp;greylisting&nbsp;policies&nbsp;<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-common-mistakes-to-avoid\"><strong>Common Mistakes to&nbsp;Avoid<\/strong>&nbsp;<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-resending-immediately-without-waiting\">Resending&nbsp;immediately&nbsp;without waiting&nbsp;<\/li>\n\n\n\n<li id=\"htoc-ignoring-batch-limits\">Ignoring batch limits&nbsp;<\/li>\n\n\n\n<li id=\"htoc-misinterpreting-soft-bounces-as-permanent-failures\">Misinterpreting soft bounces as permanent failures&nbsp;<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-empowering-insight\"><strong>Empowering Insight<\/strong>&nbsp;<\/h2>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"aligncenter size-full\"><img loading=\"lazy\" decoding=\"async\" width=\"1600\" height=\"300\" src=\"https:\/\/1grid.co.za\/knowledge\/wp-content\/uploads\/2026\/03\/1-grid-Images-for-KB-Articles-1000-x-600-px-1600-x-300-px.png\" alt=\"Empowering Insight\" class=\"wp-image-34464\"\/><\/figure>\n<\/div>\n\n\n<p>SMTP 421 errors are usually temporary. A short wait and retry often resolves the issue without changes to your setup.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-next-recommended-steps\"><strong>Next Recommended Steps<\/strong>&nbsp;<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-pause-sending-and-wait-15-to-30-minutes\">Pause sending and wait 15 to 30 minutes&nbsp;<\/li>\n\n\n\n<li id=\"htoc-review-delivery-logs\">Review delivery logs&nbsp;<\/li>\n\n\n\n<li id=\"htoc-adjust-sending-batch-sizes-if-high-volume\">Adjust sending batch sizes if high-volume&nbsp;<\/li>\n\n\n\n<li id=\"htoc-retry-sending-to-deferred-recipients\">Retry sending to deferred recipients&nbsp;<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-faqs\"><strong>FAQs<\/strong>&nbsp;<\/h2>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Q. Is SMTP 421 permanent?&nbsp;<\/strong><\/summary>\n<p id=\"htoc-q-is-smtp-421-permanent-no-it-is-a-temporary-rejection-messages-are-typically-retried-automatically\">No, it is a temporary rejection. Messages are typically retried automatically.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Q. Does this mean my domain is blocked?<\/strong><\/summary>\n<p id=\"htoc-q-does-this-mean-my-domain-is-blocked-not-usually-as-421-errors-are-temporary-and-usually-server-side\">Not usually, as 421 errors are temporary and usually server-side.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Q. Can I fix this myself?&nbsp;<\/strong><\/summary>\n<p id=\"htoc-q-can-i-fix-this-myself-yes-by-waiting-checking-logs-and-adjusting-batch-sending-contact-support-if-persistent\">Yes, by waiting, checking logs, and adjusting batch sending. Contact support if persistent.&nbsp;<\/p>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-additional-resources\"><strong>Additional&nbsp;Resources<\/strong>&nbsp;<\/h2>\n\n\n\n<p><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/a-guide-to-smtperror-codes\/\" target=\"_blank\" rel=\"noreferrer noopener\">SMTP Error Codes<\/a>\u00a0\u00a0<\/strong><br><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/how-to-enable-dkim-and-spf-on-your-mail-domain\/\" target=\"_blank\" rel=\"noreferrer noopener\">SPF\/DKIM Setup Guide<\/a>\u00a0<\/strong>\u00a0<br><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/email-restrictions\/\" target=\"_blank\" rel=\"noreferrer noopener\">Email Sending Limits Guide<\/a>\u00a0<\/strong>\u00a0<br><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/why-emails-bounce-back-error-messages-and-how-to-fix-them\/\" target=\"_blank\" rel=\"noreferrer noopener\">Email Bounce Back Guide<\/a>\u00a0<\/strong><\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-need-additional-support\"><strong>Need Additional Support?<\/strong>&nbsp;<\/h2>\n\n\n\n<p><strong>We&#8217;re&nbsp;Here to&nbsp;<\/strong><a href=\"https:\/\/1grid.co.za\/knowledge\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Help<\/strong><\/a><strong>:<\/strong>&nbsp;<\/p>\n\n\n\n<p>Understanding SMTP Error&nbsp;421&nbsp;and how to fix this&nbsp;doesn\u2019t&nbsp;have&nbsp;to&nbsp;feel technical&nbsp;with&nbsp;this&nbsp;easy-to-reference guide. Stuck?&nbsp;Check out our&nbsp;<a href=\"https:\/\/1grid.co.za\/knowledge\/1-grid-scope-of-support\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Scope of Support<\/strong><\/a>, and then contact our Support Team for&nbsp;further&nbsp;clarity and&nbsp;guidance&nbsp;(<a href=\"https:\/\/1grid.co.za\/contact-us\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>https:\/\/1grid.co.za\/contact-us\/<\/strong><\/a>).&nbsp;We&#8217;re&nbsp;ready&nbsp;to see&nbsp;how we can help!&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>If&nbsp;you\u2019ve&nbsp;received&nbsp;SMTP Error 421, it means the receiving mail server is temporarily unavailable or too busy to process your email.&nbsp; This guide will:&nbsp; By the end of this guide,&nbsp;you\u2019ll&nbsp;confidently manage 421 errors without unnecessary support escalation.&nbsp; What SMTP Error&nbsp;421&nbsp;Means&nbsp; SMTP 421 is a&nbsp;temporary rejection (soft bounce).&nbsp; Common scenarios include:&nbsp; Unlike hard&#8230;<\/p>\n","protected":false},"author":21,"comment_status":"closed","ping_status":"closed","template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"ht-kb-category":[1194],"ht-kb-tag":[1728,1726,1725,1727,1494,1707,1514,1715,1706,1724,1701],"class_list":["post-34547","ht_kb","type-ht_kb","status-publish","format-standard","hentry","ht_kb_category-email-troubleshooting","ht_kb_tag-421-greylisting","ht_kb_tag-421-temporary-resource-limits","ht_kb_tag-421-temporary-server-unavailable","ht_kb_tag-421-timeout-or-connection-limits","ht_kb_tag-email-configuration","ht_kb_tag-email-delivery-issues","ht_kb_tag-email-troubleshooting","ht_kb_tag-smtp-421-fix","ht_kb_tag-smtp-bounce-errors","ht_kb_tag-smtp-error-421","ht_kb_tag-smtp-errors"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.0 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>SMTP Error 421: Temporary Server Unavailable (How to Fix It) - 1-grid Knowledge Base<\/title>\n<meta name=\"description\" content=\"Fix SMTP Error 421: Temporary Server Unavailable. 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