SMTP bounce codes and what they mean

An SMTP bounce code tells you why a receiving mail server refused or delayed your message. Every reply starts with a three-digit code (RFC 5321 §4.2), and most servers add an enhanced status code of the form class.subject.detail (RFC 3463). The first digit already answers the most important question: 4 means try again later, 5 means do not.

Providers add their own wording and sometimes their own meaning for a code, so each entry below is tied to the provider that documents it. Paste a whole bounce message into the lookup tool to have the code and provider recognised for you.

Bounce code lookup (paste a message)Paste a bounce message or enter an SMTP code such as 550 5.7.26 to see what it means at Gmail, Microsoft 365 or Yahoo, why it happens and how to fix it. Runs in your browser.

Standard SMTP and enhanced codes (RFC 5321, RFC 3463)

• 421 SMTP: service not available
• 450 SMTP: mailbox unavailable
• 451 SMTP: local processing error
• 452 SMTP: insufficient storage
• 550 SMTP: mailbox unavailable
• 551 SMTP: user not local
• 552 SMTP: storage exceeded
• 553 SMTP: mailbox name not allowed
• 554 SMTP: transaction failed
• 5.1.1 SMTP: bad destination mailbox
• 5.1.2 SMTP: bad destination system
• 5.1.3 SMTP: bad address syntax
• 5.1.10 SMTP: recipient has null MX
• 4.2.2 SMTP: mailbox full
• 5.2.2 SMTP: mailbox full
• 5.2.3 SMTP: message too large
• 5.3.4 SMTP: message too big for system
• 4.4.1 SMTP: no answer from host
• 5.4.4 SMTP: unable to route
• 5.7.0 SMTP: security status
• 4.7.0 SMTP: security status
• 5.7.1 SMTP: delivery not authorized
• 5.7.23 SMTP: SPF validation failed
• 5.7.25 SMTP: reverse DNS failed
• 5.7.26 SMTP: authentication failed
• 5.7.27 SMTP: sender has null MX

Gmail

• 550 5.7.26 Gmail: blocked by dmarc policy
• 550 5.7.26 Gmail: no spf or dkim pass
• 550 5.7.26 Gmail: SPF hard fail
• 550 5.7.27 Gmail: SPF not passed
• 550 5.7.25 Gmail: no PTR record
• 421 4.7.0 Gmail: no PTR record
• 451 4.7.23 Gmail: PTR record missing
• 421 4.7.28 Gmail: rate limited
• 550 5.7.1 Gmail: likely unsolicited
• 550 5.7.30 Gmail: DKIM not passed
• 550 5.7.40 Gmail: no DMARC record

Microsoft 365 and Outlook.com

• 550 5.1.1 Microsoft: bad mailbox address
• 550 5.1.10 Microsoft: recipient not found
• 5.4.1 Microsoft: relay access denied
• 5.4.1 Microsoft: recipient rejected
• 5.4.14 Microsoft: routing loop
• 550 5.5.0 Microsoft: mailbox unavailable
• 550 5.7.1 Microsoft: delivery not authorized
• 550 5.7.1 Microsoft: unable to relay
• 530 5.7.1 Microsoft: not authenticated
• 5.7.23 Microsoft: SPF violation
• 5.7.25 Microsoft: IPv6 needs reverse DNS
• 5.7.57 Microsoft: anonymous send blocked
• 5.7.509 Microsoft: DMARC reject
• 5.7.705 Microsoft: tenant over threshold
• 5.7.708 Microsoft: IP not accepted

Yahoo

• 554 Yahoo: account does not exist