{"id":77,"date":"2026-02-22T14:54:18","date_gmt":"2026-02-22T14:54:18","guid":{"rendered":"https:\/\/mail-tester.am\/blog\/?p=77"},"modified":"2026-03-04T10:27:29","modified_gmt":"2026-03-04T10:27:29","slug":"soft-vs-hard-bounces","status":"publish","type":"post","link":"https:\/\/mail-tester.com\/blog\/soft-vs-hard-bounces\/","title":{"rendered":"Soft vs. Hard Bounces"},"content":{"rendered":"\n<p>Email bounces occur when the receiving server rejects a message. The bounce report usually contains an SMTP status code and a diagnostic string, which together indicate whether the failure is temporary or permanent. Understanding these codes allows you to maintain list hygiene, tune retry logic, and protect deliverability.<br><br>This article explains soft vs hard bounces, interprets SMTP 4xx and 5xx codes, and provides examples from Gmail and Microsoft 365.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"h-the-core-difference\"><strong>The Core Difference<\/strong><\/h2>\n\n\n\n<figure class=\"wp-block-image size-large\"><img fetchpriority=\"high\" decoding=\"async\" width=\"1024\" height=\"693\" src=\"https:\/\/mail-tester.am\/blog\/wp-content\/uploads\/2026\/02\/image-7-1024x693.png\" alt=\"\" class=\"wp-image-197\" srcset=\"https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image-7-1024x693.png 1024w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image-7-300x203.png 300w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image-7-768x520.png 768w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image-7-1536x1040.png 1536w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image-7.png 1968w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><\/figure>\n\n\n\n<p><strong>4xx = Soft bounce (temporary)<\/strong><strong><br><\/strong>The server says: \u201cNot now. Try later.\u201d<br>These are transient issues such as rate limiting, DNS hiccups, temporary blocks, infrastructure delays, or mailbox over-quota conditions.<\/p>\n\n\n\n<p><strong>5xx = Hard bounce (permanent)<\/strong><strong><br><\/strong>The server says: \u201cThis will not work as-is.\u201d<br>These indicate invalid recipients, authentication failures, policy rejection, or domain\/IP reputation problems.<\/p>\n\n\n\n<p>MTAs retry 4xx for hours but stop immediately on 5xx unless infrastructure changes.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"h-soft-bounce-4xx-examples\"><strong>Soft Bounce (4xx) Examples<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-gmail-soft-bounces\"><strong>Gmail Soft Bounces<\/strong><\/h3>\n\n\n\n<p>Common temporary Gmail conditions include: \u2022 <strong>421 4.3.0 \/ 4.4.5 \/ 4.7.0<\/strong> \u2013 Temporary system problem or server busy<br>\u2022 <strong>450 4.2.1 \/ 4.7.1<\/strong> \u2013 Relay limit exceeded, recipient policy deferral<br>\u2022 <strong>452 4.2.2<\/strong> \u2013 Mailbox full<br>\u2022 <strong>450 4.2.0 \/ 451 4.3.0<\/strong> \u2013 Mailbox or local resource temporarily unavailable<br>\u2022 <strong>421 4.7.0<\/strong> \u2013 Temporary anti-abuse or reputation-based deferral<br>\u2022 <strong>421 4.7.0<\/strong> \u2013 TLS required but temporarily unavailable<\/p>\n\n\n\n<p>Gmail uses 421, 450, and 452 for rate limits, temporary blocks, and infrastructure instability.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-microsoft-365-soft-bounces\"><strong>Microsoft 365 Soft Bounces<\/strong><\/h3>\n\n\n\n<p>Exchange Online uses 4xx codes for transient conditions: \u2022 <strong>421 4.4.1 \/ 4.3.2<\/strong> \u2013 Timeout or service unavailable<br>\u2022 <strong>450 4.2.0<\/strong> \u2013 Mailbox temporarily unavailable<br>\u2022 <strong>452 4.2.2 \/ 4.3.1<\/strong> \u2013 Mailbox full or system storage limits<br>\u2022 <strong>451 4.7.0 \/ 4.7.500<\/strong> \u2013 Temporary policy or server busy<br>\u2022 <strong>450 4.4.312 \/ 451 4.4.0<\/strong> \u2013 Temporary DNS lookup failure<\/p>\n\n\n\n<p>These should be retried, with reduced concurrency if deferrals repeat.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"h-hard-bounce-5xx-examples\"><strong>Hard Bounce (5xx) Examples<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-gmail-hard-bounces\"><strong>Gmail Hard Bounces<\/strong><\/h3>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-invalid-recipients-5-1-x\"><strong>Invalid recipients (5.1.x)<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>550 5.1.1 \/ 5.1.0 \/ 5.1.10<\/strong> \u2013 Recipient does not exist<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-mailbox-disabled-or-over-quota-5-2-x\"><strong>Mailbox disabled or over quota (5.2.x)<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>550 5.2.1<\/strong> \u2013 Disabled mailbox<br>\u2022 <strong>552 5.2.2 \/ 5.2.3<\/strong> \u2013 Quota exceeded or message too large<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-authentication-policy-rejection-5-7-x\"><strong>Authentication \/ policy rejection (5.7.x)<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>550 5.7.26<\/strong> \u2013 DMARC rejection<br>\u2022 <strong>550 5.7.1<\/strong> \u2013 Domain policy or content-based block<br>\u2022 <strong>550 5.7.0<\/strong> \u2013 Unauthorized mail relay<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-content-and-formatting-failures\"><strong>Content and formatting failures<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>554 5.5.0 \/ 553 5.3.0<\/strong> \u2013 Format or address syntax issues<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"h-microsoft-365-hard-bounces\"><strong>Microsoft 365 Hard Bounces<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-invalid-recipients-5-1-x-0\"><strong>Invalid recipients (5.1.x)<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>550 5.1.1 \/ 5.1.10<\/strong> \u2013 User not found<br>\u2022 <strong>551 5.1.x<\/strong> \u2013 User not local<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-mailbox-disabled-or-restricted-5-2-x\"><strong>Mailbox disabled or restricted (5.2.x)<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>550 5.2.1<\/strong> \u2013 Disabled mailbox<br>\u2022 <strong>552 5.2.2<\/strong> \u2013 Quota exceeded<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-authentication-and-policy-failures-5-7-x\"><strong>Authentication and policy failures (5.7.x)<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>550 5.7.1<\/strong> \u2013 Not permitted to send as this sender<br>\u2022 <strong>5.7.57<\/strong> \u2013 Client not authenticated<br>\u2022 <strong>550 5.7.606 \/ 5.7.708<\/strong> \u2013 IP banned or blocked<br>\u2022 <strong>550 5.7.23<\/strong> \u2013 SPF failure<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-routing-and-content-issues\"><strong>Routing and content issues<\/strong><\/h3>\n\n\n\n<p>\u2022 <strong>553 5.5.4<\/strong> \u2013 Invalid domain<br>\u2022 <strong>554 5.4.14<\/strong> \u2013 Routing loop detected<br>\u2022 <strong>554 5.5.0<\/strong> \u2013 Message refused by policy<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img decoding=\"async\" width=\"1024\" height=\"663\" src=\"https:\/\/mail-tester.am\/blog\/wp-content\/uploads\/2026\/02\/image2-1024x663.png\" alt=\"\" class=\"wp-image-198\" srcset=\"https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image2-1024x663.png 1024w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image2-300x194.png 300w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image2-768x497.png 768w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image2-1536x994.png 1536w, https:\/\/mail-tester.com\/blog\/wp-content\/uploads\/2026\/02\/image2.png 1792w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><\/figure>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-soft-bounces-4xx\"><strong>Soft Bounces (4xx)<\/strong><\/h3>\n\n\n\n<p>\u2022 Retry with exponential backoff<br>\u2022 Reduce concurrency for throttled domains<br>\u2022 Watch domain-specific rate limits<br>\u2022 Maintain stable sending patterns<br>\u2022 Warm IPs and sending domains carefully<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"h-hard-bounces-5xx\"><strong>Hard Bounces (5xx)<\/strong><\/h3>\n\n\n\n<p>Treat all hard bounces as permanent unless the provider explicitly states otherwise.<\/p>\n\n\n\n<p id=\"h-immediately-suppress-5-1-x-invalid-users-suppress-5-2-1-disabled-mailboxes-optionally-retry-once-for-5-2-2-5-2-3-do-not-retry-5-7-x-until-authentication-or-policy-issues-are-fixed\"><strong>List hygiene:<br><\/strong>\u2022 Immediately suppress <strong>5.1.x<\/strong> invalid users<br>\u2022 Suppress <strong>5.2.1<\/strong> disabled mailboxes<br>\u2022 Optionally retry once for <strong>5.2.2 \/ 5.2.3<br><\/strong>\u2022 Do not retry <strong>5.7.x<\/strong> until authentication or policy issues are fixed<\/p>\n\n\n\n<p><strong>Infrastructure:<\/strong><strong><br><\/strong>\u2022 Fix SPF, DKIM, DMARC alignment<br>\u2022 Repair connectors or relay permissions<br>\u2022 Resolve domain\/IP reputation issues<br>\u2022 Send small verification batches before resuming full traffic<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"h-organizational-level-governance\"><strong>Organizational-Level Governance<\/strong><\/h2>\n\n\n\n<p>Hard bounce remediation should include:<br>\u2022 Global suppression rules across all systems<br>\u2022 Monitoring clusters of hard bounces for acquisition or list-quality issues<br>\u2022 Authentication and policy hardening for all sending sources<br>\u2022 Reviewing spam\/content rejection patterns<br>\u2022 Enforcing verification and sunset policies to prevent decay of list quality<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"h-summary\"><strong>Summary<\/strong><\/h2>\n\n\n\n<p>To sum it up, soft bounces indicate temporary conditions that resolve with proper retry logic and pacing. Hard bounces point to permanent issues such as invalid recipients or authentication and policy failures, and should feed directly into suppression and infrastructure corrections.<\/p>\n\n\n\n<p>When SMTP codes are interpreted correctly, they expose where the failure sits: the recipient address, the remote server, your own authentication setup, or your sending behavior. This allows you to adjust retry windows, maintain list accuracy, and reinforce SPF, DKIM, and DMARC alignment.<\/p>\n\n\n\n<p>Bounce data is not noise. It is a diagnostic layer that helps maintain a stable sender reputation and a predictable delivery path.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Email bounces occur when the receiving server rejects a message. The bounce report usually contains an SMTP status code and a diagnostic string, which together indicate whether the failure is temporary or permanent. Understanding these codes allows you to maintain list hygiene, tune retry logic, and protect deliverability. This article explains soft vs hard bounces, [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":199,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[],"class_list":["post-77","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-email-deliverability"],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.2 (Yoast SEO v27.2) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Email Bounce Codes: SMTP 4xx vs 5xx | Mail-Tester<\/title>\n<meta name=\"description\" content=\"Learn soft vs hard bounces and decode SMTP 4xx\/5xx errors with Gmail and Microsoft 365 examples. 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