{"id":5724,"date":"2022-01-24T07:56:22","date_gmt":"2022-01-24T13:56:22","guid":{"rendered":"https:\/\/www.knownhost.com\/kb\/?p=5724"},"modified":"2022-08-23T09:21:13","modified_gmt":"2022-08-23T14:21:13","slug":"smtp-error-421","status":"publish","type":"post","link":"https:\/\/www.knownhost.com\/kb\/smtp-error-421\/","title":{"rendered":"SMTP Error 421"},"content":{"rendered":"<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_82_2 ez-toc-wrap-right counter-hierarchy ez-toc-counter ez-toc-custom ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #212121;color:#212121\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #212121;color:#212121\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 eztoc-toggle-hide-by-default' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.knownhost.com\/kb\/smtp-error-421\/#what_does_this_mean\" >What does this mean?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.knownhost.com\/kb\/smtp-error-421\/#how_to_fix_smtp_error_421\" >How to fix SMTP Error 421?<\/a><\/li><\/ul><\/nav><\/div>\n\n<p><strong>Description<\/strong>: Service not available, closing transmission channel.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"what_does_this_mean\"><\/span>What does this mean?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>This is an unfortunately, open ended error. Most commonly this is in relation to the destination server not being &#8220;reachable&#8221;, however it may also mean that your sending mail server is not &#8220;reachable&#8221;. The latter of the two options is less common, unless a remote MX exchange is being used. The error message is also an indication that the message will continue being attempted for delivery.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"how_to_fix_smtp_error_421\"><\/span>How to fix SMTP Error 421?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>If you are using a remote mail server for sending email, you&#8217;ll want to ensure that it&#8217;s functioning properly and connections to it are successful.<\/p>\n\n\n\n<p>If you are not using a remote mail server (or it&#8217;s working properly) you&#8217;ll want to check the destination server&#8217;s availibilty. Check out the&nbsp;<span style=\"text-decoration: underline\">Testing Remote SMTP \/ Mail Server Connections<\/span>&nbsp;article referenced at the end of this article.<\/p>\n\n\n\n<p><strong>Related<\/strong> <strong>article<\/strong>: <a href=\"https:\/\/www.knownhost.com\/kb\/testing-remote-smtp-mail-server-connections\/\">Testing Remote SMTP \/ Mail Server Connections<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Description: Service not available, closing transmission channel. What does this mean? This is an unfortunately, open ended error. Most commonly this is in relation to the destination server not being &#8220;reachable&#8221;, however it may also mean that your sending mail server is not &#8220;reachable&#8221;. The latter of the two options is less common, unless a [&hellip;]<\/p>\n","protected":false},"author":6,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[97],"tags":[554,380,291],"class_list":["post-5724","post","type-post","status-publish","format-standard","hentry","category-other","tag-554","tag-error","tag-smtp"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SMTP Error 421 - KnownHost<\/title>\n<meta name=\"description\" content=\"KnownHost self-hosted email guide to help you troubleshoot and fix SMTP Error 421 on your premium KnownHost server.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.knownhost.com\/kb\/smtp-error-421\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"SMTP Error 421 - 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