{"id":3985,"date":"2025-01-20T18:23:16","date_gmt":"2025-01-20T18:23:16","guid":{"rendered":"https:\/\/www.warmy.io\/blog\/smtp-error-codes-and-messages\/"},"modified":"2026-06-24T11:41:11","modified_gmt":"2026-06-24T11:41:11","slug":"smtp-error-codes-and-messages","status":"publish","type":"post","link":"https:\/\/www.warmy.io\/blog\/smtp-error-codes-and-messages\/","title":{"rendered":"SMTP Error Codes and Messages: What Every Code Means and How to Fix"},"content":{"rendered":"\n

When an email fails to reach its destination, your mail server records exactly what went wrong in the form of an SMTP error code. Each code is a standardized three-digit number defined in RFC 5321<\/a>, the Internet Engineering Task Force specification that governs how email servers communicate. The first digit tells you the outcome class: 2 for success, 3 for a pending action, 4 for a temporary failure, and 5 for a permanent one. The second and third digits give you the specific cause.<\/p>\n\n\n\n

Most delivery problems trace back to one of three root causes: a misconfigured sending domain (missing SPF, DKIM, or DMARC records), a damaged sender reputation, or a transient server-side condition on the recipient’s end. Knowing which class of code you are dealing with tells you immediately whether to retry, fix your configuration, or investigate your domain health. According to Gmail’s bulk sender guidelines<\/a>, senders are required to keep spam complaint rates below 0.3% and must authenticate all outgoing mail with SPF, DKIM, and DMARC \u2014 making proper configuration the single most important step in avoiding 4XX and 5XX errors.<\/p>\n\n\n\n

This is the complete directory of SMTP error codes and messages, organized by class, with plain-English explanations and actionable fixes for every code you are likely to encounter. If a 4XX code is slowing down your campaigns, or a 5XX is blocking delivery entirely, the answer is here.<\/p>\n\n\n\n

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