{"id":34682,"date":"2026-06-09T10:45:30","date_gmt":"2026-06-09T10:45:30","guid":{"rendered":"https:\/\/1grid.co.za\/knowledge\/?post_type=ht_kb&#038;p=34682"},"modified":"2026-06-09T10:52:04","modified_gmt":"2026-06-09T10:52:04","slug":"smtp-error-553-invalid-email-address-format-how-to-fix","status":"publish","type":"ht_kb","link":"https:\/\/1grid.co.za\/knowledge\/smtp-error-553-invalid-email-address-format-how-to-fix\/","title":{"rendered":"SMTP Error 553: Invalid Email Address Format (How to Fix)"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">SMTP Error <strong>553<\/strong> occurs when an email server <strong>rejects a message because the sender or recipient address is invalid, incorrectly formatted, or not authorised for relay<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This guide will help you:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-understand-why-the-553-error-occurs\">Understand why the 553 error occurs<\/li>\n\n\n\n<li id=\"htoc-identify-the-specific-variation-returned-by-the-server\">Identify the specific variation returned by the server<\/li>\n\n\n\n<li id=\"htoc-correct-address-formatting-or-authentication-issues\">Correct address formatting or authentication issues<\/li>\n\n\n\n<li id=\"htoc-restore-successful-email-delivery\">Restore successful email delivery<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Most SMTP 553 errors are <strong>simple formatting or configuration mistake<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">By the end of this guide, you\u2019ll confidently diagnose and resolve SMTP 553 errors without unnecessary support escalation.<\/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\/06\/1-grid-Images-for-KB-Articles-1000-x-600-px-22.png\" alt=\"SMTP Error 553: Invalid Email Address Format (How to Fix)\" class=\"wp-image-34683\"\/><\/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-step-by-step-fix-for-smtp-553-error-invalid-email-address-format\">Step-by-Step Fix for SMTP 553 Error \u2013 Invalid Email Address Format<\/a><\/li><li class=\"\"><a href=\"#htoc-quick-fixes-for-common-smtp-554-variations\">Quick Fixes for Common SMTP 554 Variations&nbsp;<\/a><\/li><li class=\"\"><a href=\"#htoc-advanced-optional-fixes-for-technical-users\">Advanced (Optional) Fixes (For Technical Users)<\/a><\/li><\/ul><\/div><\/div>\n\n\n\n<h2 id=\"htoc-what-smtp-error-553-means\" class=\"wp-block-heading\">What SMTP Error 553 Means<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">SMTP 553 is a <strong>permanent email rejection (<a href=\"https:\/\/1grid.co.za\/knowledge\/why-emails-bounce-back-error-messages-and-how-to-fix-them\/#htoc-hard-bounce-permanent-failure\">hard bounc<\/a><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\">e<\/a>)<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The receiving server refuses the message because it cannot accept the email address or sender identity.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This typically happens when:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-the-email-address-format-is-invalid\">The email address format is invalid<\/li>\n\n\n\n<li id=\"htoc-the-recipient-does-not-exist\">The recipient does not exist<\/li>\n\n\n\n<li id=\"htoc-the-sending-server-requires-authentication\">The sending server requires authentication<\/li>\n\n\n\n<li id=\"htoc-the-domain-is-not-recognised\">The domain is not recognised<\/li>\n\n\n\n<li id=\"htoc-relay-permissions-are-restricted\">Relay permissions are restricted<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Because it is a <strong>permanent failure<\/strong>, the server <strong>will not retry delivery automatically<\/strong>.<\/p>\n\n\n\n<h2 id=\"htoc-why-this-matters\" class=\"wp-block-heading\">Why This Matters<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Ignoring SMTP 553 errors may lead to:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-failed-email-delivery\">Failed email delivery<\/li>\n\n\n\n<li id=\"htoc-broken-contact-forms\">Broken contact forms<\/li>\n\n\n\n<li id=\"htoc-increased-bounce-rates\">Increased bounce rates<\/li>\n\n\n\n<li id=\"htoc-reduced-sender-reputation\">Reduced sender reputation<\/li>\n\n\n\n<li id=\"htoc-automated-email-system-failures\">Automated email system failures<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Correctly identifying the cause ensures reliable email delivery without impacting your hosting environment.<\/p>\n\n\n\n<h2 id=\"htoc-step-by-step-fix-for-smtp-553-error-invalid-email-address-format\" class=\"wp-block-heading\"><strong>Step-by-Step Fix for SMTP 553 Error \u2013 Invalid Email Address Format<\/strong><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Follow these steps in order.<\/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<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-open-the-bounce-message-and-confirm-it-includes-variations-such-as\">Open the bounce message and confirm it includes variations such as:<\/p>\n\n\n\n<pre id=\"htoc-553-invalid-user\" class=\"wp-block-code\"><code>553 Invalid user\n553 5.7.1 Not an open relay\n553 sorry, no mailbox here by that name\n553 user unknown\n553 address does not exist\n553 domain of sender address does not exist<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-if-a-different-smtp-code-appears-refer-to-the-smtp-error-codes-guide\">If a different SMTP code appears, refer to our\u00a0<strong><a href=\"https:\/\/1grid.co.za\/knowledge\/a-guide-to-smtperror-codes\/\" target=\"_blank\" rel=\"noreferrer noopener\">SMTP Error Codes<\/a>\u00a0<\/strong>guide.<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>2: Verify the Email Address Format<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-confirm-the-email-address-includes\">Confirm the email address includes:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-username-domain-com\"><a href=\"mailto:username@domain.com\">username@domain.com<\/a><\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-check-for\">Check for:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-missing-symbol\">missing <strong>@ symbol<\/strong><\/li>\n\n\n\n<li id=\"htoc-missing-domain\">missing domain<\/li>\n\n\n\n<li id=\"htoc-spaces-in-the-address\">spaces in the address<\/li>\n\n\n\n<li id=\"htoc-unsupported-characters\">unsupported characters<\/li>\n\n\n\n<li id=\"htoc-double-periods\">double periods<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-correct-the-address-and-resend-the-email\">Correct the address and resend the email.<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>3: Confirm the Recipient Address Exists<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-contact-the-recipient-to-verify\">Contact the recipient to verify:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-the-email-address-spelling\">the email address spelling<\/li>\n\n\n\n<li id=\"htoc-whether-the-mailbox-is-still-active\">whether the mailbox is still active<\/li>\n\n\n\n<li id=\"htoc-if-the-email-address-has-changed\">if the email address has changed<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-update-the-address-before-retrying-the-delivery\">Update the address before retrying the delivery.<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>4: Enable SMTP Authentication<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-some-servers-return-smtp-553-when-authentication-is-required-but-not-enabled\">Some servers return SMTP 553 when <strong>authentication is required but not enabled<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-check-your-email-client-settings\">Check your<a href=\"https:\/\/1grid.co.za\/knowledge\/settings-to-configure-your-1grid-email-accounts-across-devices\/\"><strong> <\/strong><\/a><a href=\"https:\/\/1grid.co.za\/knowledge\/settings-to-configure-your-1grid-email-accounts-across-devices\/\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>email client settings<\/strong><\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-ensure\">Ensure:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-smtp-authentication-enabled\">SMTP Authentication: Enabled<\/li>\n\n\n\n<li id=\"htoc-use-same-credentials-as-incoming-mail-server\">Use same credentials as incoming mail server<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-without-authentication-many-servers-will-reject-outgoing-mail\">Without authentication, many servers will reject outgoing mail.<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>5: Check the Sender Domain<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-some-553-errors-occur-if-the-sender-domain\">Some 553 errors occur if the sender domain:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-does-not-exist\">does not exist<\/li>\n\n\n\n<li id=\"htoc-is-incorrectly-typed\">is incorrectly typed<\/li>\n\n\n\n<li id=\"htoc-lacks-dns-records\">lacks <a href=\"https:\/\/1grid.co.za\/knowledge\/understanding-dns-records-and-how-to-check-these\/\"><strong>DNS records<\/strong><\/a><\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-verify-that-the-sender-s-email-domain-is-valid-and-active\">Verify that the sender&#8217;s email domain is valid and active.<\/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 Level)<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-log-in-to\">Log in to:<\/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>\u00a0<\/strong>\u2192 Email \u2192\u00a0<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>\u00a0<\/strong>\u2192\u00a0Websites &amp; Domains \u2192 Select your domain \u2192 Mail \u2192 Track Email Delivery\u00a0<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-confirm\">Confirm:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-the-server-returned-a-553-permanent-rejection\">the server returned a <strong>553 permanent rejection<\/strong><\/li>\n\n\n\n<li id=\"htoc-the-exact-variation-returned-by-the-recipient-server\">the <strong>exact variation returned by the recipient server<\/strong><\/li>\n\n\n\n<li id=\"htoc-the-timestamp-and-delivery-attempt-details\">the <strong>timestamp and delivery attempt details<\/strong><\/li>\n<\/ul>\n<\/details>\n\n\n\n<h2 id=\"htoc-quick-fixes-for-common-smtp-554-variations\" class=\"wp-block-heading\">Quick Fixes for Common SMTP 554 Variations&nbsp;<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">SMTP 553 errors are <strong>permanent rejections<\/strong>, meaning the message cannot be delivered until the issue is corrected.<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>1. 553 5.7.1 Not an Open Relay<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-example-553-5-7-1-not-an-open-relay553-authentication-required\">Example:<\/p>\n\n\n\n<pre id=\"htoc-example-553-5-7-1-not-an-open-relay553-authentication-required1\" class=\"wp-block-code\"><code>553 5.7.1 Not an open relay\n553 authentication required<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-what-it-means-the-mail-server-requires-authentication-before-sending-messages\"><strong>What It Means:<\/strong><br>The mail server requires authentication before sending messages.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-how-to-fix-it\"><strong>How to Fix It:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-enable-smtp-authentication-in-your-email-client\">Enable<strong> <a href=\"https:\/\/1grid.co.za\/knowledge\/outlook-how-to-enable-smtp-authentication\/\" target=\"_blank\" rel=\"noreferrer noopener\">SMTP authentication<\/a><\/strong> in your email client<\/li>\n\n\n\n<li id=\"htoc-use-your-full-email-address-as-the-username\">Use your full email address as the username<\/li>\n\n\n\n<li id=\"htoc-enter-the-correct-mailbox-password\">Enter the correct mailbox password<\/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. 553 Invalid User<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-example-553-invalid-user553-user-unknown\">Example:<\/p>\n\n\n\n<pre id=\"htoc-example-553-invalid-user553-user-unknown1\" class=\"wp-block-code\"><code>553 invalid user\n553 user unknown<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-what-it-means-the-recipient-mailbox-does-not-exist\"><strong>What It Means:<\/strong><br>The recipient mailbox does not exist.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-how-to-fix-it1\"><strong>How to Fix It:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-confirm-the-recipient-email-address-spelling\">Confirm the recipient email address spelling<\/li>\n\n\n\n<li id=\"htoc-ask-the-recipient-to-verify-the-correct-address\">Ask the recipient to verify the correct address<\/li>\n\n\n\n<li id=\"htoc-remove-invalid-addresses-from-mailing-lists\">Remove invalid addresses from mailing lists<\/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. 553 Domain of Sender Address Does Not Exist<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-example-553-5-1-8-domain-of-sender-address-does-not-exist\">Example:<\/p>\n\n\n\n<pre id=\"htoc-example-553-5-1-8-domain-of-sender-address-does-not-exist1\" class=\"wp-block-code\"><code>553 #5.1.8 Domain of sender address does not exist<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-what-it-means-the-sending-email-domain-cannot-be-validated\"><strong>What It Means:<\/strong><br>The sending email domain cannot be validated.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-how-to-fix-it11\"><strong>How to Fix It:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-verify-the-sender-domain-spelling\">Verify the sender domain spelling<\/li>\n\n\n\n<li id=\"htoc-ensure-the-domain-is-active\">Ensure the <a href=\"https:\/\/1grid.co.za\/knowledge\/customer-zone-how-to-view-all-your-domains\/\"><strong>domain is active<\/strong><\/a><\/li>\n\n\n\n<li id=\"htoc-confirm-dns-records-exist\">Confirm DNS records exist<\/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>4. 553 Invalid Email Address Format<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-example-553-address-syntax-error\">Example:<\/p>\n\n\n\n<pre id=\"htoc-example-553-address-syntax-error1\" class=\"wp-block-code\"><code>553 address syntax error<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-what-it-means-the-email-address-format-does-not-meet-server-validation-rules\"><strong>What It Means:<\/strong><br>The email address format does not meet server validation rules.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-how-to-fix-it111\"><strong>How to Fix It:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-remove-spaces-or-special-characters\">Remove spaces or special characters<\/li>\n\n\n\n<li id=\"htoc-ensure-the-address-format-is-correct\">Ensure the address format is correct<\/li>\n\n\n\n<li id=\"htoc-re-enter-the-address-manually\">Re-enter the address manually<\/li>\n<\/ul>\n<\/details>\n\n\n\n<h2 id=\"htoc-advanced-optional-fixes-for-technical-users\" class=\"wp-block-heading\">Advanced (Optional) Fixes (For Technical Users)<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Administrators or Technical Users can diagnose authentication or relay issues at the server-level.<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>1. Check Exim Mail Logs<\/strong><\/summary>\n<pre id=\"htoc-grep-553-var-log-exim-mainlog\" class=\"wp-block-code\"><code>grep \"553\" \/var\/log\/exim_mainlog<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-look-for\">Look for:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-relay-denial-responses\">relay denial responses<\/li>\n\n\n\n<li id=\"htoc-invalid-sender-domains\">invalid sender domains<\/li>\n\n\n\n<li id=\"htoc-authentication-failures\">authentication failures<\/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. Verify Sender Domain DNS<\/strong><\/summary>\n<pre id=\"htoc-dig-example-com-mxdig-example-com-txt\" class=\"wp-block-code\"><code>dig example.com MX<br>dig example.com TXT<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-confirm1\">Confirm:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-domain-resolves-correctly\">domain resolves correctly<\/li>\n\n\n\n<li id=\"htoc-spf-records-exist\">SPF records exist<\/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. Test SMTP Authentication via Telnet<\/strong><\/summary>\n<pre id=\"htoc-telnet-mail-domain-com-25\" class=\"wp-block-code\"><code>telnet mail.domain.com 25<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-then\">Then:<\/p>\n\n\n\n<pre id=\"htoc-ehlo-domain-comauth-login\" class=\"wp-block-code\"><code>EHLO domain.com<br>AUTH LOGIN<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-confirm-that-authentication-is-accepted\">Confirm that authentication is accepted.<\/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 Mail Server Relay Restrictions<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-in-mail-server-configuration\">In mail server configuration:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"htoc-verify\">Verify:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-smtp-authentication-enabled1\">SMTP authentication enabled<\/li>\n\n\n\n<li id=\"htoc-sender-domain-policies\">sender domain policies<\/li>\n\n\n\n<li id=\"htoc-relay-restrictions\">relay restrictions<\/li>\n<\/ul>\n<\/details>\n\n\n\n<h2 id=\"htoc-before-you-move-on\" class=\"wp-block-heading\">Before You Move On<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-confirm-the-email-address-format-is-valid\">Confirm the email address format is valid<\/li>\n\n\n\n<li id=\"htoc-verify-the-recipient-address-exists\">Verify the recipient address exists<\/li>\n\n\n\n<li id=\"htoc-ensure-smtp-authentication-is-enabled\">Ensure SMTP authentication is enabled<\/li>\n\n\n\n<li id=\"htoc-confirm-the-sender-domain-is-active\">Confirm the sender domain is active<\/li>\n<\/ul>\n\n\n\n<h2 id=\"htoc-if-this-didn-t-work\" class=\"wp-block-heading\">If This Didn\u2019t Work<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Try these steps:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-send-a-test-email-to-another-address\">Send a test email to another address<\/li>\n\n\n\n<li id=\"htoc-re-enter-the-email-address-manually\">Re-enter the email address manually<\/li>\n\n\n\n<li id=\"htoc-verify-dns-records-for-the-sender-domain\">Verify DNS records for the sender domain<\/li>\n\n\n\n<li id=\"htoc-review-delivery-logs-for-additional-errors\">Review delivery logs for additional errors<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">If the problem persists, contact support with:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-full-bounce-message\">full bounce message<\/li>\n\n\n\n<li id=\"htoc-timestamp\">timestamp<\/li>\n\n\n\n<li id=\"htoc-sender-address\">sender address<\/li>\n\n\n\n<li id=\"htoc-recipient-address\">recipient address<\/li>\n<\/ul>\n\n\n\n<h2 id=\"htoc-common-mistakes-to-avoid\" class=\"wp-block-heading\">Common Mistakes to Avoid<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li id=\"htoc-sending-emails-without-authentication\">Sending emails without authentication<\/li>\n\n\n\n<li id=\"htoc-copy-pasting-addresses-with-hidden-characters\">Copy-pasting addresses with hidden characters<\/li>\n\n\n\n<li id=\"htoc-using-invalid-sender-domains\">Using invalid sender domains<\/li>\n\n\n\n<li id=\"htoc-sending-to-outdated-mailing-list-contacts\">Sending to outdated mailing list contacts<\/li>\n<\/ul>\n\n\n\n<h2 id=\"htoc-empowering-insight\" class=\"wp-block-heading\">Empowering Insight<\/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 class=\"wp-block-paragraph\">Most SMTP 553 errors are caused by <strong>simple formatting mistakes or missing authentication<\/strong>.<br><br>Correcting the address or enabling authentication typically resolves the issue immediately.<\/p>\n\n\n\n<h2 id=\"htoc-faqs\" class=\"wp-block-heading\">FAQs<\/h2>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary><strong>Q. Will SMTP 553 affect my website?<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-q-will-smtp-553-affect-my-website-no-this-error-only-affects-the-specific-email-message-being-sent\">No, this error only affects the specific email message being sent.<\/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 the server retry the message automatically?<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-q-can-the-server-retry-the-message-automatically-no-smtp-553-is-a-permanent-rejection-and-must-be-fixed-before-resending\">No, SMTP 553 is a permanent rejection and must be fixed before resending.<\/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 blacklisted?<\/strong><\/summary>\n<p class=\"wp-block-paragraph\" id=\"htoc-q-does-this-mean-my-domain-is-blacklisted-no-smtp-553-errors-are-usually-related-to-formatting-or-authentication-issues\">No, SMTP 553 errors are usually related to formatting or authentication issues.<\/p>\n<\/details>\n\n\n\n<h2 id=\"htoc-additional-resources\" class=\"wp-block-heading\">Additional Resources<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><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 id=\"htoc-need-additional-support\" class=\"wp-block-heading\">Need Additional Support?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>We&#8217;re Here to <\/strong><a href=\"https:\/\/1grid.co.za\/knowledge\/\"><strong>Help<\/strong><\/a><strong>:<\/strong> <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Understanding SMTP Error 553 and how to fix this doesn\u2019t have to feel technical with this easy-to-reference guide. Stuck? Check out our <a href=\"https:\/\/1grid.co.za\/knowledge\/1-grid-scope-of-support\/\"><strong>Scope of Support<\/strong><\/a>, and then contact our Support Team for further clarity and guidance (<a href=\"https:\/\/1grid.co.za\/contact-us\/\"><strong>https:\/\/1grid.co.za\/contact-us\/<\/strong><\/a>). We&#8217;re ready to see how we can help!<a id=\"_msocom_1\"><\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>SMTP Error 553 occurs when an email server rejects a message because the sender or recipient address is invalid, incorrectly formatted, or not authorised for relay. This guide will help you: Most SMTP 553 errors are simple formatting or configuration mistake. By the end of this guide, you\u2019ll confidently diagnose&#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":[1772,1771,1765,1769,1687,1770],"class_list":["post-34682","ht_kb","type-ht_kb","status-publish","format-standard","hentry","ht_kb_category-email-troubleshooting","ht_kb_tag-553-authentication-required","ht_kb_tag-553-invalid-email-address","ht_kb_tag-email-bounce-error","ht_kb_tag-email-delivery-error","ht_kb_tag-smtp-error","ht_kb_tag-smtp-error-553-fix"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.0 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>SMTP Error 553: Invalid Email Address Format (How to Fix) - 1-grid Knowledge Base<\/title>\n<meta name=\"description\" content=\"Fix SMTP Error 553 invalid email address format. 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