Email Knowledge Bounce codes guide

Bounce codes, explained

Bounces are delivery failure reports. This guide helps you quickly find a code, understand what it usually means, and pick a sensible next step. Use the search box to filter instantly.

Find a bounce code

Many bounces contain both a three-digit SMTP reply code (like 550) and an enhanced status code (like 5.1.1). If you see an enhanced code, it is often the most useful starting point.

Popular codes

Tip: Codes in the table are clickable when a detailed page is available.

Example snippets

Action: failed
Status: 5.1.1
Diagnostic-Code: smtp; 550 5.1.1 User unknown
Action: delayed
Status: 4.7.1
Diagnostic-Code: smtp; 451 4.7.1 Try again later
Action: failed
Status: 5.7.1
Diagnostic-Code: smtp; 550 5.7.1 Message rejected under local policy

What to do next

Addressing failures (often 5.1.x) typically require correcting or removing the recipient. Temporary failures (4.x.x) are often retried automatically; repeated deferrals can indicate throttling. Policy failures (often 5.7.x) frequently relate to authentication (SPF/DKIM/DMARC), reputation, or blocklists.

For the broader technical background (SMTP, DNS, SPF/DKIM/DMARC), see the main page: email-knowledge.com.