{"id":2740,"date":"2026-06-19T15:41:27","date_gmt":"2026-06-19T19:41:27","guid":{"rendered":"https:\/\/www.inmotionhosting.com\/support\/2013\/05\/28\/smtp-and-esmtp-error-code-list\/"},"modified":"2026-06-19T15:46:55","modified_gmt":"2026-06-19T19:46:55","slug":"smtp-and-esmtp-error-code-list","status":"publish","type":"post","link":"https:\/\/www.inmotionhosting.com\/support\/email\/smtp-and-esmtp-error-code-list\/","title":{"rendered":"SMTP and ESMTP Error Code List"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">When an email fails to deliver, the receiving server sends a <strong>bounce message<\/strong> (also called a <a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-bounce-bounceback-error\/\">bounceback<\/a> or delivery status notification) back to the sender. These contain <strong>SMTP error codes<\/strong> that explain why delivery failed.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">SMTP Code Classes Explained<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>2xx codes<\/strong>: Success (you usually won\u2019t see these in bounces).<\/li>\n\n\n\n<li><strong>3xx codes<\/strong>: Intermediate (e.g., \u201cstart mail input\u201d).<\/li>\n\n\n\n<li><strong>4xx codes<\/strong>: <strong>Temporary \/ Soft bounce<\/strong>. The server will usually retry automatically. Common with greylisting, temporary server load, or network issues.<\/li>\n\n\n\n<li><strong>5xx codes<\/strong>: <strong>Permanent \/ Hard bounce<\/strong>. Delivery will not be retried. You must fix the underlying issue (bad address, blocked, policy violation, etc.).<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Extended SMTP (ESMTP) codes<\/strong> appear in the format <code>X.Y.Z<\/code> (e.g., <code>5.1.1<\/code>):<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>First digit = Class (2 = success, 4 = temporary, 5 = permanent)<\/li>\n\n\n\n<li>Second digit = Subject category<\/li>\n\n\n\n<li>Third digit = Specific detail<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Many bounces show both, for example: <code>550 5.1.1 User unknown<\/code><\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Most Common SMTP Errors and How to Resolve Them on InMotion Hosting<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">SMTP Error 550 \u2013 Requested Action Not Taken: Mailbox Unavailable \/ Invalid Address<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Also appears as:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>550 No such user here<\/code><\/li>\n\n\n\n<li><code>550 User unknown<\/code><\/li>\n\n\n\n<li><code>550 Recipient does not exist<\/code><\/li>\n\n\n\n<li><code>550 invalid address<\/code><\/li>\n\n\n\n<li><code>550 relaying denied<\/code><\/li>\n\n\n\n<li><code>5.1.1 Bad destination mailbox address<\/code><\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>What It Means:<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This is a <strong>permanent (5xx)<\/strong> error. The receiving mail server does not recognize the recipient address or will not accept mail for it.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Common Causes:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Typo in the recipient\u2019s email address.<\/li>\n\n\n\n<li>The recipient\u2019s email account was deleted or never existed.<\/li>\n\n\n\n<li>Recent DNS\/MX record changes (domain moved or MX updated).<\/li>\n\n\n\n<li>The recipient\u2019s server has strict policies or the address is on a suppression list.<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">How to Fix (If You Are the Sender):<\/h4>\n\n\n\n<ol class=\"wp-block-list article_list\">\n<li>Carefully check the recipient address for typos.<\/li>\n\n\n\n<li>Contact the recipient through another method (phone, alternate email) and confirm their current email address.<\/li>\n\n\n\n<li>Ask them to check if the account still exists on their end.<\/li>\n<\/ol>\n\n\n\n<h4 class=\"wp-block-heading\">How to Fix (If Others Are Bouncing When Sending to You):<\/h4>\n\n\n\n<ol class=\"wp-block-list article_list\">\n<li>Log into cPanel \u2192 <strong>Email Accounts<\/strong> and confirm the address exists and is active.<\/li>\n\n\n\n<li>Check MX records in cPanel \u2192 <strong>Zone Editor<\/strong> or <strong>MX Entry<\/strong> to ensure they point to InMotion\u2019s mail servers.<\/li>\n\n\n\n<li>If you recently transferred the domain or changed nameservers\/MX, allow 24\u201348 hours for propagation and re-test.<\/li>\n\n\n\n<li>Create the missing email account if needed.<\/li>\n<\/ol>\n\n\n\n<p class=\"wp-block-paragraph\">If the problem persists for legitimate addresses, <a href=\"https:\/\/www.inmotionhosting.com\/support\/amp\/submitting-and-checking-on-status-of-tickets\/\">submit a support ticket<\/a> with the full bounce message.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">SMTP Error 535 \u2013 Authentication Required \/ Incorrect Authentication<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Common Variations:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>535 Incorrect authentication data<\/code><\/li>\n\n\n\n<li><code>535 5.7.8 Authentication failed<\/code><\/li>\n\n\n\n<li>multiple servers using same IP requiring auth<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>What It Means:<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The sending server requires authentication that was not provided or was incorrect. On InMotion VPS\/Dedicated servers, repeated 535 errors in Exim logs often indicate brute-force login attempts by spammers.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>For Shared Hosting Customers:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Ensure <strong>\u201cMy outgoing server requires authentication\u201d<\/strong> is enabled in your email client.<\/li>\n\n\n\n<li>Use the full email address and correct password.<\/li>\n\n\n\n<li>If you recently changed your email password, update it in all email clients and apps (including phones).<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>For VPS or Dedicated Server Customers (Advanced):<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">You can investigate failed login attempts in the Exim mail log:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>grep \"535 Incorrect\" \/var\/log\/exim_mainlog | awk -F\"set_id=\" '{print $2}' | sort | uniq -c | sort -n<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Identify offending IPs and block them at the firewall (e.g., using <code>firewalld<\/code>, <code>ufw<\/code>, or your preferred tool). Contact InMotion support if you need assistance interpreting logs or implementing rate limiting\/Fail2Ban.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">SMTP Error 421 \u2013 Service Not Available \/ Temporary Failure<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Common Variations:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>421 Service not available<\/code><\/li>\n\n\n\n<li><code>421 Too many connections<\/code><\/li>\n\n\n\n<li><code>421 Greylisting enabled<\/code><\/li>\n\n\n\n<li><code>please try later<\/code><\/li>\n\n\n\n<li><code>4.7.1<\/code><\/li>\n\n\n\n<li><code>4.4.5<\/code><\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>What It Means:<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This is a <strong>temporary (4xx)<\/strong> error. The receiving server is currently unable or unwilling to accept the message (busy, greylisting, connection limits, temporary resource issue). Delivery is usually retried automatically.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>How to Fix:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>In most cases, <strong>do nothing<\/strong> as the sending server will retry.<\/li>\n\n\n\n<li>If you see many 421 errors in your mail logs or repeated soft bounces:<\/li>\n\n\n\n<li>Check for high outbound email volume (newsletters, marketing).<\/li>\n\n\n\n<li>Review your sending practices (avoid spammy content, ensure proper authentication\/SPF\/DKIM).<\/li>\n\n\n\n<li>Contact the recipient\u2019s postmaster if the issue persists for important mail.<\/li>\n\n\n\n<li>For InMotion customers seeing widespread 421s when sending: Submit a ticket so our team can review outbound delivery health.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Other Common 5xx Permanent Errors<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>552 \u2013 Requested mail action aborted: exceeded storage allocation<\/strong><br>Mailbox is full on the recipient\u2019s end. Ask the recipient to delete old messages or increase quota. If this is your own mailbox, check cPanel \u2192 Email Accounts \u2192 Quota or contact support.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>553 \u2013 Requested action not taken: mailbox name not allowed<\/strong><br>Often caused by invalid characters in the local part of the address or policy restrictions on the receiving server. Verify the address format.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>554 \u2013 Transaction failed<\/strong><br>Generic permanent failure. Could be content filtering (spam\/virus), policy violation, or DNS issues. Review the full bounce message for more clues. Test with a simple plain-text email.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>450 \u2013 Requested mail action not taken: mailbox unavailable<\/strong><br>Temporary (4xx) version of mailbox issues. Usually retries. Can also indicate greylisting or the recipient server is temporarily rejecting.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Full SMTP Error Code Reference<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">200\u2013300 Series (Informational \/ Success)<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>211<\/strong> \u2013 System status \/ help reply<\/li>\n\n\n\n<li><strong>214<\/strong> \u2013 Help message<\/li>\n\n\n\n<li><strong>220<\/strong> \u2013 Service ready<\/li>\n\n\n\n<li><strong>221<\/strong> \u2013 Service closing transmission channel<\/li>\n\n\n\n<li><strong>250<\/strong> \u2013 Requested mail action completed, OK<\/li>\n\n\n\n<li><strong>354<\/strong> \u2013 Start mail input; end with <code>&lt;CRLF>.&lt;CRLF><\/code><\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">400 Series (Temporary Failures \u2013 Usually Retried)<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-error-421\/\"><strong>421<\/strong> \u2013 Service not available<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-error-450\/\"><strong>450<\/strong> \u2013 Mailbox unavailable<\/a> (temporary)<\/li>\n\n\n\n<li><strong>451<\/strong> \u2013 Local error in processing; try again later<\/li>\n\n\n\n<li><strong>452<\/strong> \u2013 Insufficient system storage<\/li>\n\n\n\n<li><strong>454<\/strong> \u2013 TLS not available (temporary)<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">500 Series (Permanent Failures \u2013 Action Required)<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>500<\/strong> \u2013 Syntax error, command unrecognized<\/li>\n\n\n\n<li><strong>501<\/strong> \u2013 Syntax error in parameters<\/li>\n\n\n\n<li><strong>502<\/strong> \u2013 Command not implemented<\/li>\n\n\n\n<li><strong>503<\/strong> \u2013 Bad sequence of commands<\/li>\n\n\n\n<li><strong>504<\/strong> \u2013 Command parameter not implemented<\/li>\n\n\n\n<li><strong>510 \/ 512<\/strong> \u2013 Bad recipient address or domain not found<\/li>\n\n\n\n<li><strong>530<\/strong> \u2013 Access denied \/ Authentication required<\/li>\n\n\n\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-error-535\/\"><strong>535<\/strong> \u2013 Authentication failed<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/550-invalid-address\/\"><strong>550<\/strong> \u2013 Mailbox unavailable \/ Invalid address<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-error-551\/\"><strong>551<\/strong> \u2013 User not local; try forward path<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-error-552\/\"><strong>552<\/strong> \u2013 Exceeded storage allocation (mailbox full)<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-error-553\/\"><strong>553<\/strong> \u2013 Mailbox name not allowed<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.inmotionhosting.com\/support\/email\/email-error-554\/\"><strong>554<\/strong> \u2013 Transaction failed<\/a><\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">Extended SMTP (ESMTP) Status Codes (x.y.z)<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">These provide more granular detail:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>x.1.x \u2013 Address Status<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>X.1.1<\/code> \u2013 Bad destination mailbox address (very common with 550)<\/li>\n\n\n\n<li><code>X.1.3<\/code> \u2013 Bad destination mailbox address syntax<\/li>\n\n\n\n<li><code>X.1.7<\/code> \u2013 Bad sender\u2019s mailbox address syntax<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>x.2.x \u2013 Mailbox Status<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>X.2.1<\/code> \u2013 Mailbox disabled, not accepting messages<\/li>\n\n\n\n<li><code>X.2.2<\/code> \u2013 Mailbox full<\/li>\n\n\n\n<li><code>X.2.3<\/code> \u2013 Message length exceeds limit<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>x.3.x \u2013 Mail System Status<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>X.3.1<\/code> \u2013 Mail system full<\/li>\n\n\n\n<li><code>X.3.4<\/code> \u2013 Message too big for system<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>x.4.x \u2013 Network\/Routing<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>X.4.1<\/code> \u2013 No answer from host<\/li>\n\n\n\n<li><code>X.4.2<\/code> \u2013 Bad connection<\/li>\n\n\n\n<li><code>X.4.7<\/code> \u2013 Delivery time expired (message too old)<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>x.5.x \u2013 Protocol<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>X.5.1<\/code> \u2013 Invalid command<\/li>\n\n\n\n<li><code>X.5.2<\/code> \u2013 Syntax error<\/li>\n\n\n\n<li><code>X.5.3<\/code> \u2013 Too many recipients<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>x.7.x \u2013 Security\/Policy<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>X.7.1<\/code> \u2013 Delivery not authorized, message refused (spam\/policy block)<\/li>\n\n\n\n<li><code>X.7.2<\/code> \u2013 Mailing list expansion prohibited<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>When an email fails to deliver, the receiving server sends a bounce message (also called a bounceback or delivery status notification) back to the sender. These contain SMTP error codes that explain why delivery failed. SMTP Code Classes Explained Extended SMTP (ESMTP) codes appear in the format X.Y.Z (e.g., 5.1.1): Many bounces show both, for<a class=\"moretag\" href=\"https:\/\/www.inmotionhosting.com\/support\/email\/smtp-and-esmtp-error-code-list\/\"> Read More ><\/a><\/p>\n","protected":false},"author":56983,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[4],"tags":[],"class_list":["post-2740","post","type-post","status-publish","format-standard","hentry","category-email"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SMTP and ESMTP Error Code List - Understanding Email Bounces<\/title>\n<meta name=\"description\" content=\"Learn what Standard SMTP error codes and the Extended SMTP (ESMTP) error codes mean for email bounce diagnostics.\" \/>\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.inmotionhosting.com\/support\/email\/smtp-and-esmtp-error-code-list\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"SMTP and ESMTP Error Code List - Understanding Email Bounces\" \/>\n<meta property=\"og:description\" content=\"Learn what Standard SMTP error codes and the Extended SMTP (ESMTP) error codes mean for email bounce diagnostics.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.inmotionhosting.com\/support\/email\/smtp-and-esmtp-error-code-list\/\" \/>\n<meta property=\"og:site_name\" content=\"InMotion Hosting Support Center\" \/>\n<meta property=\"article:publisher\" content=\"https:\/\/www.facebook.com\/inmotionhosting\/\" \/>\n<meta property=\"article:published_time\" content=\"2026-06-19T19:41:27+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-06-19T19:46:55+00:00\" \/>\n<meta name=\"author\" content=\"Carrie Smaha\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:creator\" content=\"@carriesmaha\" \/>\n<meta name=\"twitter:site\" content=\"@InMotionHosting\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Carrie Smaha\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"5 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/email\\\/smtp-and-esmtp-error-code-list\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/email\\\/smtp-and-esmtp-error-code-list\\\/\"},\"author\":{\"name\":\"Carrie Smaha\",\"@id\":\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/#\\\/schema\\\/person\\\/595948dab2995d347a87076abdae19d8\"},\"headline\":\"SMTP and ESMTP Error Code List\",\"datePublished\":\"2026-06-19T19:41:27+00:00\",\"dateModified\":\"2026-06-19T19:46:55+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/email\\\/smtp-and-esmtp-error-code-list\\\/\"},\"wordCount\":961,\"commentCount\":0,\"publisher\":{\"@id\":\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/#organization\"},\"articleSection\":[\"Email\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/email\\\/smtp-and-esmtp-error-code-list\\\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/email\\\/smtp-and-esmtp-error-code-list\\\/\",\"url\":\"https:\\\/\\\/www.inmotionhosting.com\\\/support\\\/email\\\/smtp-and-esmtp-error-code-list\\\/\",\"name\":\"SMTP and ESMTP Error Code List - 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