{"id":114,"date":"2026-10-06T13:21:37","date_gmt":"2026-10-06T10:21:37","guid":{"rendered":"https:\/\/mx1mail.com\/blog\/email-bounces-and-smtp-error-codes-explained\/"},"modified":"2026-10-06T13:21:37","modified_gmt":"2026-10-06T10:21:37","slug":"email-bounces-and-smtp-error-codes-explained","status":"publish","type":"post","link":"https:\/\/mx1mail.com\/blog\/en\/email-bounces-and-smtp-error-codes-explained\/","title":{"rendered":"Email Bounces and SMTP Error Codes Explained"},"content":{"rendered":"<h2>What is an email bounce?<\/h2>\n<p>When an email cannot be delivered to the destination mail server or recipient mailbox, the sending system may receive an error notification. These notices are commonly called bounce messages or delivery status notifications.<\/p>\n<h2>What do 4xx SMTP codes mean?<\/h2>\n<p>SMTP codes beginning with 4 generally indicate temporary problems. Server load, temporary connectivity issues or greylisting may cause the sending server to retry delivery later.<\/p>\n<h2>What do 5xx SMTP codes mean?<\/h2>\n<p>Codes beginning with 5 generally indicate permanent delivery failures. The recipient may not exist, the message may violate a policy or the destination domain may be unable to accept delivery.<\/p>\n<h2>Understanding error 550<\/h2>\n<p>550 is one of the most common SMTP error codes, but the number alone does not identify the exact problem. The additional text usually explains whether the user does not exist, the message was rejected or another policy prevented delivery.<\/p>\n<h2>Mailbox full errors<\/h2>\n<p>If the recipient has reached the mailbox storage limit, a message may be temporarily or permanently rejected. Monitoring mailbox quotas is therefore important in business email environments.<\/p>\n<h2>Spam or policy rejection<\/h2>\n<p>A receiving server may reject a message because of suspected spam, IP reputation, SPF, DKIM, DMARC results or content policies. Administrators should examine the full error and server logs rather than simply sending the same message again.<\/p>\n<h2>Keep the error message<\/h2>\n<p>When investigating delivery issues, details such as the SMTP error code, destination server, timestamp and Message-ID can make technical diagnosis much easier.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Understand email bounce messages and the basic difference between temporary 4xx and permanent 5xx SMTP delivery errors.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[217],"tags":[254,258,256],"class_list":["post-114","post","type-post","status-publish","format-standard","hentry","category-email-infrastructure","tag-bounce","tag-email-delivery","tag-smtp-error-codes"],"_links":{"self":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/posts\/114","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/comments?post=114"}],"version-history":[{"count":0,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/posts\/114\/revisions"}],"wp:attachment":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/media?parent=114"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/categories?post=114"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/tags?post=114"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}