550 No Such User: Causes, Solutions, and How to Fix It
Code550 Bounce typeHard bounce RetryableNo Action neededRemove addressTypical error message 550 No such userWhat does 550 No such user mean?The receiving server has ...
SMTP Codes
454 4.7.1 Relay Access Denied: Causes, Solutions, and How to Fix It
Code454 4.7.1 Bounce typeSoft bounce RetryableYes (after fix) Action neededFix relay authTypical error message 454 4.7.1 Relay access deniedWhat does 454 4.7.1 Relay access ...
452 4.2.2 The Recipient’s Inbox Is Out of Storage Space: Causes and Fix
Code452 4.2.2 Bounce typeSoft bounce RetryableYes (limited) Action neededRetry, then suppressTypical error message 452 4.2.2 The recipient's inbox is out of storage space. Please ...
451 Relay Not Permitted: Causes, Solutions, and How to Fix It
Code451 Bounce typeSoft bounce RetryableYes (after fix) Action neededFix relay configTypical error message 451 relay not permitted!What does 451 relay not permitted mean?The ...
451 Greylisted, Please Try Again in 60 Seconds: Causes and Fix
Code451 Bounce typeSoft bounce RetryableYes Action neededWait 60s, retryTypical error message 451 Greylisted, please try again in 60 secondsWhat does 451 Greylisted mean?The ...
450 4.2.0 Recipient Address Rejected Greylisted: Causes and Fix
Code450 4.2.0 Bounce typeSoft bounce RetryableYes Action neededWait and retryTypical error message 450 4.2.0 Recipient address rejected: GreylistedWhat does 450 4.2.0 ...
450 4.7.1 Client Host Rejected: Cannot Find Your Hostname
Code450 4.7.1 Bounce typeSoft bounce RetryableYes (after fix) Action neededFix reverse DNSTypical error message 450 4.7.1 Client host rejected: cannot find your hostname,What ...
421 Service Temporarily Unavailable: Causes, Solutions, and How to Fix It
Code421 Bounce typeSoft bounce RetryableYes Action neededRetry laterTypical error message 421 Service Temporarily UnavailableWhat does 421 Service Temporarily Unavailable mean? ...
SMTP 550 5.7.1 Error: Meaning, Causes, and How to Fix It
550 5.7.1 is a permanent SMTP rejection from the receiver's policy layer. Fix authentication (SPF, DKIM, DMARC), PTR records, and reputation with provider-specific steps for Gmail, Outlook 365, and ...
SMTP Error Codes: Complete Reference Directory (160+ codes documented)
This directory documents every SMTP error code you are likely to encounter, from common 421 soft bounces to obscure server-specific 554 rejections from Nemesis, Comcast cmsmtp, GoDaddy osplibsmtp, ...
