{"id":34485,"date":"2026-03-10T11:38:09","date_gmt":"2026-03-10T11:38:09","guid":{"rendered":"https:\/\/1grid.co.za\/knowledge\/?post_type=ht_kb&#038;p=34485"},"modified":"2026-03-25T12:53:26","modified_gmt":"2026-03-25T12:53:26","slug":"a-guide-to-smtp-error-codes","status":"publish","type":"ht_kb","link":"https:\/\/1grid.co.za\/knowledge\/a-guide-to-smtperror-codes\/","title":{"rendered":"A Guide to SMTP\u00a0Error Codes\u00a0"},"content":{"rendered":"\n<p>SMTP error codes do not automatically mean your email, domain or server is broken &#8211; most are configuration or recipient-side issues.&nbsp;<\/p>\n\n\n\n<p>This <strong>guide will help you understand SMTP&nbsp;error&nbsp;codes<\/strong> and what they actually mean.&nbsp;<\/p>\n\n\n\n<p>By the end,&nbsp;you\u2019ll&nbsp;be able to&nbsp;identify&nbsp;whether an error is temporary or permanent and apply the correct next step confidently.&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-1-2.png\" alt=\"SMTP and ESMTP Codes\" class=\"wp-image-34491\"\/><\/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\">Quick Links<\/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-is-an-smtp-status-code\">What Is an SMTP Status Code?&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-smtp-error-codes\">SMTP Error Codes&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-most-common-smtp-errors-and-quick-fixes\">Most Common SMTP Errors (And Quick Fixes)&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-understanding-extended-smtp-codes-5-7-1-5-1-1-etc\">Understanding Extended SMTP Codes (5.7.1, 5.1.1, etc.)&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-advanced-optional-for-technical-users\">Advanced&nbsp;(Optional)&nbsp;(For Technical Users)&nbsp;<\/a><\/li><\/ul><\/div><\/div>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-what-is-an-smtp-status-code\"><strong>What Is an SMTP Status Code?<\/strong>&nbsp;<\/h2>\n\n\n\n<p>When an email&nbsp;fails to&nbsp;send or deliver, the mail server responds with a numeric code.&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>What These Codes Indicate<\/strong><\/summary>\n<p id=\"htoc-these-codes-tell-you\"><strong>These codes tell you:&nbsp;<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Whether the message was accepted&nbsp;<\/li>\n\n\n\n<li id=\"htoc-whether-the-delivery-will-retry\">Whether the delivery will retry&nbsp;<\/li>\n\n\n\n<li id=\"htoc-whether-it-permanently-failed\">Whether it permanently failed&nbsp;<\/li>\n\n\n\n<li id=\"htoc-why-it-failed\">Why it failed&nbsp;<\/li>\n<\/ul>\n<\/details>\n\n\n\n<ul class=\"wp-block-list\"><\/ul>\n\n\n\n<p>Understanding the <em>first digit <\/em>is the key.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-smtp-error-codes\"><strong>SMTP Error Codes<\/strong>&nbsp;<\/h2>\n\n\n\n<p>This list of codes&nbsp;is&nbsp;the standard SMTP error codes you will find in most email bounces.&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>200 Codes&nbsp;&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>211 \u2013 System status\/system&nbsp;help&nbsp;reply&nbsp;<\/li>\n\n\n\n<li>214 \u2013 Help message&nbsp;<\/li>\n\n\n\n<li>220 \u2013 Domain service ready&nbsp;<\/li>\n\n\n\n<li>221 \u2013 Domain service closing transmission channel&nbsp;<\/li>\n\n\n\n<li>250 \u2013 Requested mail action completed and OK&nbsp;<\/li>\n\n\n\n<li>251 \u2013 Not Local User, forward email to&nbsp;forward&nbsp;path&nbsp;<\/li>\n\n\n\n<li>252 \u2013 Cannot verify user, will&nbsp;attempt&nbsp;delivery later&nbsp;<\/li>\n\n\n\n<li>253 \u2013 Pending messages for node started<\/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>300 Codes<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>354 \u2013 Start mail input; end&nbsp;with .&nbsp;<\/li>\n\n\n\n<li>355 \u2013 Octet-offset is the transaction offset&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>400 Codes&nbsp;&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>421 \u2013 Domain service not available, closing transmission channel&nbsp;<\/li>\n\n\n\n<li>432 \u2013 Domain service not available, closing transmission channel&nbsp;<\/li>\n\n\n\n<li>450 \u2013 Requested mail action not taken: mailbox unavailable. request refused&nbsp;<\/li>\n\n\n\n<li>451 \u2013 Requested action aborted: local error in processing. Request is unable to be processed, try again&nbsp;<\/li>\n\n\n\n<li>452 \u2013 Requested action not taken: insufficient system storage&nbsp;<\/li>\n\n\n\n<li>453 \u2013 No mail&nbsp;<\/li>\n\n\n\n<li>454 \u2013 TLS&nbsp;not&nbsp;available due to a temporary&nbsp;reason. Encryption is required for the requested authentication mechanism&nbsp;<\/li>\n\n\n\n<li>458 \u2013 Unable to queue messages for node&nbsp;<\/li>\n\n\n\n<li>459 \u2013 Node not allowed: reason&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>500 Codes&nbsp;&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>500 \u2013 Syntax error, command unrecognized&nbsp;<\/li>\n\n\n\n<li>501 \u2013 Syntax error in parameters or arguments&nbsp;<\/li>\n\n\n\n<li>502 \u2013 Command not implemented&nbsp;<\/li>\n\n\n\n<li>503 \u2013 Bad sequence of commands&nbsp;<\/li>\n\n\n\n<li>504 \u2013 Command parameter not implemented&nbsp;<\/li>\n\n\n\n<li>510 \u2013 Check the recipient address&nbsp;<\/li>\n\n\n\n<li>512 \u2013 Domain&nbsp;can not&nbsp;be found. Unknown host.&nbsp;<\/li>\n\n\n\n<li>515 \u2013 Destination&nbsp;mailbox&nbsp;address invalid&nbsp;<\/li>\n\n\n\n<li>517 \u2013 Problem with&nbsp;sender&#8217;s&nbsp;mail attribute, check properties&nbsp;<\/li>\n\n\n\n<li>521 \u2013 Domain does not accept mail&nbsp;<\/li>\n\n\n\n<li>522 \u2013 Recipient has exceeded mailbox limit&nbsp;<\/li>\n\n\n\n<li>523 \u2013 Server limit exceeded. Message too large&nbsp;<\/li>\n\n\n\n<li>530 \u2013 Access Denied. Authentication required&nbsp;<\/li>\n\n\n\n<li>531 \u2013 Mail system Full&nbsp;<\/li>\n\n\n\n<li>533 \u2013 Remote server has insufficient disk space to hold email&nbsp;<\/li>\n\n\n\n<li>534 \u2013 Authentication mechanism is too weak. Message too big&nbsp;<\/li>\n\n\n\n<li>535 \u2013 Multiple servers using&nbsp;same&nbsp;IP. Required Authentication&nbsp;<\/li>\n\n\n\n<li>538 \u2013 Encryption required for requested authentication mechanism&nbsp;<\/li>\n\n\n\n<li>540 \u2013 Email address has no DNS Server&nbsp;<\/li>\n\n\n\n<li>541 \u2013 No response from host&nbsp;<\/li>\n\n\n\n<li>542 \u2013 Bad Connection&nbsp;<\/li>\n\n\n\n<li>543 \u2013 Routing server failure. No available route&nbsp;<\/li>\n\n\n\n<li>546 \u2013 Email looping&nbsp;<\/li>\n\n\n\n<li>547 \u2013 Delivery time-out&nbsp;<\/li>\n\n\n\n<li>550 \u2013 Requested action not taken: mailbox unavailable&nbsp;<\/li>\n\n\n\n<li>551 \u2013 User not local; please try forward path&nbsp;<\/li>\n\n\n\n<li>552 \u2013 Requested mail action aborted: exceeded storage allocation&nbsp;<\/li>\n\n\n\n<li>553 \u2013 Requested action not taken: mailbox name not allowed&nbsp;<\/li>\n\n\n\n<li>554 \u2013 Transaction failed&nbsp;<\/li>\n<\/ul>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-most-common-smtp-errors-and-quick-fixes\"><strong>Most Common SMTP Errors (And Quick Fixes)<\/strong>&nbsp;<\/h2>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>550 \u2013 Mailbox Unavailable\u00a0<\/strong><\/summary>\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>What This Means<\/strong><\/summary>\n<p id=\"htoc-meaning-the-recipient-address-does-not-exist-or-cannot-accept-mail\">The recipient address does not exist or cannot accept mail.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Quick Fix<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li id=\"htoc-check-spelling\">Check spelling&nbsp;<\/li>\n\n\n\n<li id=\"htoc-confirm-the-mailbox-exists\">Confirm the mailbox exists&nbsp;<\/li>\n\n\n\n<li id=\"htoc-verify-dns-mx-records\">Verify DNS MX records&nbsp;<\/li>\n<\/ul>\n\n\n\n<p>Explore our full guides<strong> <a href=\"https:\/\/1grid.co.za\/knowledge\/smtp-error-550-message-rejected-by-server-how-to-fix-it\/\" target=\"_blank\" rel=\"noreferrer noopener\">here<\/a> <\/strong>for step-by-step fixes for this SMTP error.<\/p>\n<\/details>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>530 \u2013 Authentication Required\u00a0<\/strong><\/summary>\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>What This Means<\/strong><\/summary>\n<p id=\"htoc-meaning-your-email-client-is-not-authenticating-before-sending\">Your email client is not authenticating before sending.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Quick Fix&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li id=\"htoc-enable-smtp-authentication\">Enable SMTP authentication&nbsp;<\/li>\n\n\n\n<li id=\"htoc-use-the-full-email-address-as-the-username\">Use the full email address as the username&nbsp;<\/li>\n\n\n\n<li id=\"htoc-confirm-the-correct-password\">Confirm the correct password<\/li>\n<\/ul>\n<\/details>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>421 \u2013 Service Not Available\u00a0<\/strong><\/summary>\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>What This Means<\/strong><\/summary>\n<p id=\"htoc-421-service-not-available\">Temporary mail server issue.\u00a0<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Quick Fix<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li id=\"htoc-wait-and-retry\">Wait and retry\u00a0<\/li>\n\n\n\n<li id=\"htoc-check-the-server-status\">Check the server status\u00a0<\/li>\n\n\n\n<li id=\"htoc-confirm-that-no-firewall-is-blocking\">Confirm that no\u00a0firewall\u00a0is blocking\u00a0<\/li>\n<\/ul>\n\n\n\n<p>Explore our full guide<strong> <a href=\"https:\/\/1grid.co.za\/knowledge\/smtp-error-421-temporary-server-unavailable-how-to-fix-it\/\" target=\"_blank\" rel=\"noreferrer noopener\">here<\/a> <\/strong>for step-by-step fixes for this SMTP error.<\/p>\n<\/details>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>552 \u2013 Storage Exceeded\u00a0<\/strong><\/summary>\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>What This Means<\/strong><\/summary>\n<p id=\"htoc-meaning-mailbox-is-full\">Mailbox is full.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Quick Fix&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li id=\"htoc-ask-the-recipient-to-clear-the-mailbox\">Ask the recipient to clear the mailbox&nbsp;<\/li>\n\n\n\n<li id=\"htoc-reduce-attachment-size\">Reduce attachment size&nbsp;<\/li>\n<\/ul>\n<\/details>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>554 \u2013 Transaction Failed\u00a0<\/strong><\/summary>\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>What This Means<\/strong><\/summary>\n<p id=\"htoc-meaning-message-rejected-for-policy-spam-or-routing-reasons\">Message rejected for policy, spam, or routing reasons.&nbsp;<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Quick Fix&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li id=\"htoc-check-spf-dkim-records1\">Check SPF\/DKIM records&nbsp;<\/li>\n\n\n\n<li id=\"htoc-review-sending-limits\">Review sending limits&nbsp;<\/li>\n\n\n\n<li id=\"htoc-run-blacklist-check\">Run blacklist check&nbsp;<\/li>\n<\/ul>\n<\/details>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-understanding-extended-smtp-codes-5-7-1-5-1-1-etc\"><strong>Understanding Extended SMTP Codes (5.7.1, 5.1.1, etc.)<\/strong>&nbsp;<\/h2>\n\n\n\n<p>Extended codes provide more detail and work in 3-digit combinations.&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Example&nbsp;<\/strong><\/summary>\n<pre id=\"htoc-5-1-1-bad-destination-mailbox-5-7-1-delivery-not-authorised-4-4-1-temporary-routing-issue\" class=\"wp-block-code\"><code>5.1.1 \u2192 Bad destination mailbox&nbsp;<br>5.7.1 \u2192 Delivery not authorised&nbsp;<br>4.4.1 \u2192 Temporary routing issue&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>Code Pattern Explained&nbsp;<\/strong><\/summary>\n<p id=\"htoc-first-number-success-temporary-or-permanent-second-number-category-third-number-specific-reason\">First number = success, temporary, or permanent&nbsp;<br>Second number = category&nbsp;<br>Third number = specific reason&nbsp;<\/p>\n<\/details>\n\n\n\n<p>You do not need to&nbsp;memorise&nbsp;these, other than just&nbsp;identify&nbsp;whether it starts with 4 or 5, as this depends on whether&nbsp;it&#8217;s&nbsp;Temporary or Permanent.&nbsp;<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><tbody><tr><td><strong>Digit<\/strong>&nbsp;<\/td><td><strong>Meaning<\/strong>&nbsp;<\/td><td><strong>What It Means&nbsp;For&nbsp;You<\/strong>&nbsp;<\/td><\/tr><tr><td>2&nbsp;<\/td><td>Success&nbsp;<\/td><td>Email delivered&nbsp;<\/td><\/tr><tr><td>4&nbsp;<\/td><td>Temporary problem&nbsp;<\/td><td>Action&nbsp;required;&nbsp;this can be caused by a non-existent email address, a DNS problem, or your email was blocked by the receiving server.&nbsp;<\/td><\/tr><tr><td>5&nbsp;<\/td><td>Permanent&nbsp;or Fatal error&nbsp;<\/td><td>Action&nbsp;required;&nbsp;this can be caused by a non-existent email address, DNS problem, or your email was blocked by the receiving server.&nbsp;<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Quick Action Steps<\/strong><\/summary>\n<p id=\"htoc-if-the-code-starts-with\">If the code starts with:&nbsp;<\/p>\n\n\n\n<p id=\"htoc-4-wait-or-retry-5-investigate-and-fix\">4 \u2192 Wait or retry&nbsp;<br>5 \u2192 Investigate and fix&nbsp;<\/p>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-advanced-optional-for-technical-users\"><strong>Advanced&nbsp;(Optional)&nbsp;(For Technical Users)<\/strong>&nbsp;<\/h2>\n\n\n\n<p><strong>List of&nbsp;Extended&nbsp;SMTP (ESMTP) Standards<\/strong>:&nbsp;<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Address Issues \u2192&nbsp;X.1.X&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>X.1.0 \u2013 Other address status&nbsp;<\/li>\n\n\n\n<li>X.1.1 \u2013 Bad destination mailbox address&nbsp;<\/li>\n\n\n\n<li>X.1.2 \u2013 Bad destination system address&nbsp;<\/li>\n\n\n\n<li>X.1.3 \u2013 Bad destination mailbox address syntax&nbsp;<\/li>\n\n\n\n<li>X.1.4 \u2013 Destination&nbsp;mailbox&nbsp;address ambiguous&nbsp;<\/li>\n\n\n\n<li>X.1.5 \u2013 Destination&nbsp;mailbox&nbsp;address valid&nbsp;<\/li>\n\n\n\n<li>X.1.6 \u2013 Mailbox has moved&nbsp;<\/li>\n\n\n\n<li>X.1.7 \u2013 Bad sender\u2019s mailbox address syntax&nbsp;<\/li>\n\n\n\n<li>X.1.8 \u2013 Bad sender\u2019s system address&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>Mailbox Status&nbsp;\u2192 X.2.X&nbsp;&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>X.2.0 \u2013 Other or undefined mailbox status&nbsp;<\/li>\n\n\n\n<li>X.2.1 \u2013 Mailbox disabled, not accepting messages&nbsp;<\/li>\n\n\n\n<li>X.2.2 \u2013 Mailbox full&nbsp;<\/li>\n\n\n\n<li>X.2.3 \u2013 Message length exceeds administrative limit&nbsp;<\/li>\n\n\n\n<li>X.2.4 \u2013 Mailing list expansion issue<\/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>Mail System Status&nbsp;\u2192 X.3.X&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>X.3.0 \u2013 Other or undefined mail system status&nbsp;<\/li>\n\n\n\n<li>X.3.1 \u2013 Mail system full&nbsp;<\/li>\n\n\n\n<li>X.3.2 \u2013 System not accepting network messages&nbsp;<\/li>\n\n\n\n<li>X.3.3 \u2013 System not capable of selected features&nbsp;<\/li>\n\n\n\n<li>X.3.4 \u2013 Message too big for system&nbsp;<\/li>\n\n\n\n<li>X.3.5 \u2013 System incorrectly configured<\/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>Network\/Routing \u2192 X.4.X&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>X.4.0 \u2013 Other or undefined network or routing status&nbsp;<\/li>\n\n\n\n<li>X.4.1 \u2013 No answer from host&nbsp;<\/li>\n\n\n\n<li>X.4.2 \u2013 Bad connection&nbsp;<\/li>\n\n\n\n<li>X.4.3 \u2013 Routing server failure&nbsp;<\/li>\n\n\n\n<li>X.4.4 \u2013 Unable to route&nbsp;<\/li>\n\n\n\n<li>X.4.5 \u2013 Network congestion&nbsp;<\/li>\n\n\n\n<li>X.4.6 \u2013 Routing loop detected&nbsp;<\/li>\n\n\n\n<li>X.4.7 \u2013 Delivery time expired<\/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>Protocol Errors&nbsp;\u2192&nbsp;X.5.X&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li><strong>&nbsp;<\/strong>&nbsp;X.5.0 \u2013 Other or undefined protocol status&nbsp;<\/li>\n\n\n\n<li>X.5.1 \u2013 Invalid command&nbsp;<\/li>\n\n\n\n<li>X.5.2 \u2013 Syntax error&nbsp;<\/li>\n\n\n\n<li>X.5.3 \u2013 Too many recipients&nbsp;<\/li>\n\n\n\n<li>X.5.4 \u2013 Invalid command arguments&nbsp;<\/li>\n\n\n\n<li>X.5.5 \u2013 Wrong protocol version&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>Media Issues \u2192 X.6.X&nbsp;&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li><strong>&nbsp;<\/strong>&nbsp;X.6.0 \u2013 Other or undefined media error&nbsp;<\/li>\n\n\n\n<li>X.6.1 \u2013 Media not supported&nbsp;<\/li>\n\n\n\n<li>X.6.2 \u2013 Conversion required and prohibited&nbsp;<\/li>\n\n\n\n<li>X.6.3 \u2013 Conversion required but not supported&nbsp;<\/li>\n\n\n\n<li>X.6.4 \u2013 Conversion with loss performed&nbsp;<\/li>\n\n\n\n<li>X.6.5 \u2013 Conversion failed&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>Security Failures \u2192 X.7.X &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>X.7.0 \u2013 Other or undefined security status&nbsp;<\/li>\n\n\n\n<li>X.7.1 \u2013 Delivery not authorized, message refused&nbsp;<\/li>\n\n\n\n<li>X.7.2 \u2013 Mailing list expansion prohibited&nbsp;<\/li>\n\n\n\n<li>X.7.3 \u2013 Security conversion&nbsp;required&nbsp;but not possible&nbsp;<\/li>\n\n\n\n<li>X.7.4 \u2013 Security features not supported&nbsp;<\/li>\n\n\n\n<li>X.7.5 \u2013 Cryptographic failure&nbsp;<\/li>\n\n\n\n<li>X.7.6 \u2013 Cryptographic algorithm not supported&nbsp;<\/li>\n\n\n\n<li>X.7.7 \u2013 Message integrity failure&nbsp;<\/li>\n<\/ul>\n\n\n\n<p id=\"htoc-security-errors-x-7-x-often-relate-to\">Security errors (x.7.x) often relate to:&nbsp;<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>SPF failures&nbsp;<\/li>\n\n\n\n<li>DKIM failures&nbsp;<\/li>\n\n\n\n<li>DMARC policy rejection&nbsp;<\/li>\n\n\n\n<li>IP reputation&nbsp;<\/li>\n<\/ul>\n<\/details>\n\n\n\n<p>You can explore a full list of&nbsp;SMTP&nbsp;and ESMTP&nbsp;Error Codes<strong>&nbsp;<\/strong><a href=\"https:\/\/wiki.mattrude.com\/images\/0\/03\/SMTP_Server_Status_Codes_and_SMTP_Error_Codes.pdf\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>here<\/strong><\/a>.&nbsp;<\/p>\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<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Checklist<\/strong><\/summary>\n<ol class=\"wp-block-list\">\n<li>Does the code start with 4 or 5?\u00a0<\/li>\n\n\n\n<li>Is it\u00a0authentication-related?\u00a0<\/li>\n\n\n\n<li>Is it\u00a0mailbox-related?\u00a0<\/li>\n\n\n\n<li>Is it SPF\/DKIM related?\u00a0<\/li>\n<\/ol>\n<\/details>\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<ol start=\"4\" class=\"wp-block-list\"><\/ol>\n\n\n\n<p>This narrows down 90% of cases&nbsp;immediately.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-if-this-didn-t-work\"><strong>If This\u00a0Didn\u2019t\u00a0Work<\/strong><\/h2>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Check These 4 Things<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>Reset your email password\u00a0<\/li>\n\n\n\n<li>Confirm sending limits not exceeded\u00a0<\/li>\n\n\n\n<li>Verify SPF\/DKIM records\u00a0<\/li>\n\n\n\n<li>Check blacklist status\u00a0<\/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>Issues Persistent?<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li>Contact support with the full bounce message\u00a0<\/li>\n\n\n\n<li>Include the entire error string when\u00a0submitting\u00a0a ticket.\u00a0<\/li>\n<\/ul>\n<\/details>\n\n\n\n<ul class=\"wp-block-list\"><\/ul>\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<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Learn how to improve email deliverability<\/strong><\/summary>\n<ul class=\"wp-block-list\">\n<li><a href=\"https:\/\/1grid.co.za\/knowledge\/understanding-smtp-ports-and-ssltls-encryption\/\" target=\"_blank\" rel=\"noreferrer noopener\">Fix SMTP Authentication Errors\u00a0<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/1grid.co.za\/knowledge\/spf-and-dkim-records-within-cpanel\/\" target=\"_blank\" rel=\"noreferrer noopener\">Check SPF\/DKIM Records\u00a0<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/1grid.co.za\/knowledge\/email-restrictions\/\" target=\"_blank\" rel=\"noreferrer noopener\">Review Email Sending Limits<\/a>\u00a0<\/li>\n\n\n\n<li><a href=\"https:\/\/1grid.co.za\/knowledge\/how-to-check-if-my-ip-is-blacklisted\/\" target=\"_blank\" rel=\"noreferrer noopener\">Run a Blacklist Check<\/a>\u00a0<\/li>\n\n\n\n<li><a href=\"https:\/\/1grid.co.za\/knowledge\/?s=reset+email+password&amp;ht-kb-search=1&amp;lang=\" target=\"_blank\" rel=\"noreferrer noopener\">Reset Email Password\u00a0<\/a><\/li>\n<\/ul>\n<\/details>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"htoc-common-mistakes-to-avoid\"><strong>Common Mistakes to Avoid<\/strong>&nbsp;<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-assuming-every-550-error-is-a-server-failure\">Assuming every 550 error is a server failure&nbsp;<\/li>\n\n\n\n<li id=\"htoc-ignoring-authentication-requirements\">Ignoring authentication requirements&nbsp;<\/li>\n\n\n\n<li id=\"htoc-disabling-ssl-to-test\">Disabling SSL to \u201ctest\u201d&nbsp;<\/li>\n\n\n\n<li id=\"htoc-sending-high-volume-emails-without-checking-limits\">Sending high-volume emails without checking limits&nbsp;<\/li>\n\n\n\n<li id=\"htoc-editing-dns-without-confirming-records\">Editing DNS without confirming records&nbsp;<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\"><\/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-1.png\" alt=\"Empowering Insight\" class=\"wp-image-34465\"\/><\/figure>\n<\/div>\n\n\n<p>If you focus on the first digit of the code,&nbsp;you\u2019ll&nbsp;immediately&nbsp;know&nbsp;whether to wait or fix something.&nbsp;<\/p>\n\n\n\n<p>That single check&nbsp;eliminates&nbsp;most panic.&nbsp;<\/p>\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.&nbsp;Will&nbsp;an SMTP&nbsp;error affect&nbsp;my website?&nbsp;<\/strong><\/summary>\n<p id=\"htoc-no-smtp-errors-relate-only-to-email-delivery\">No,&nbsp;SMTP errors relate only to email delivery.&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.&nbsp;Can I undo&nbsp;email configuration&nbsp;changes?&nbsp;<\/strong><\/summary>\n<p id=\"htoc-yes-email-configuration-changes-can-be-reverted\">Yes, email configuration changes can be reverted.&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.&nbsp;Are 4xx errors permanent?&nbsp;<\/strong><\/summary>\n<p id=\"htoc-no-they-are-temporary-and-usually-retry-automatically\">No, they are temporary and usually retry 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.&nbsp;Does a 5xx error mean my domain is blocked?&nbsp;<\/strong><\/summary>\n<p id=\"htoc-not-necessarily-many-are-simple-configuration-issues\">Not necessarily, many are simple configuration issues.&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><a href=\"https:\/\/1grid.co.za\/knowledge\/how-to-access-your-email-via-webmail\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Accessing Your Email via Webmail<\/strong><\/a><br><a href=\"https:\/\/1grid.co.za\/knowledge\/viewingyour-email-settings-in-cpanel-and-webmail\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Viewing&nbsp;Your Email Settings in cPanel and Webmail<\/strong><\/a><br><a href=\"https:\/\/1grid.co.za\/knowledge\/viewingyour-email-settings-inwindows-plesk\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Viewing&nbsp;Your Email Settings in&nbsp;Windows Plesk<\/strong><\/a><br><a href=\"https:\/\/1grid.co.za\/knowledge\/recommended-server-settings-for-email-encryption-ports\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Recommended Server Settings for Email (Encryption &amp; Ports)&nbsp;<\/strong><\/a><br><strong><a href=\"https:\/\/1grid.co.za\/knowledge\/email-security-avoid-being-labeled-as-a-spammer\/\" target=\"_blank\" rel=\"noreferrer noopener\">Spam &amp; Reputation Guide<\/a>&nbsp;<\/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&nbsp;SMTP error codes&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!<\/p>\n","protected":false},"excerpt":{"rendered":"<p>SMTP error codes do not automatically mean your email, domain or server is broken &#8211; most are configuration or recipient-side issues.&nbsp; This guide will help you understand SMTP&nbsp;error&nbsp;codes and what they actually mean.&nbsp; By the end,&nbsp;you\u2019ll&nbsp;be able to&nbsp;identify&nbsp;whether an error is temporary or permanent and apply the correct next step&#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":[1494,1707,1514,1715,1704,1714,1713,1716,1717,1706,1708,1701],"class_list":["post-34485","ht_kb","type-ht_kb","status-publish","format-standard","hentry","ht_kb_category-email-troubleshooting","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-451-fix","ht_kb_tag-smtp-530-fix","ht_kb_tag-smtp-550-fix","ht_kb_tag-smtp-552-fix","ht_kb_tag-smtp-554-fix","ht_kb_tag-smtp-bounce-errors","ht_kb_tag-smtp-error-codes","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>A Guide to SMTP\u00a0Error Codes\u00a0 - 1-grid Knowledge Base<\/title>\n<meta name=\"description\" content=\"Understand SMTP error codes like 550, 5.7.1, and 421. 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