SMTP: 550 Mailbox not found
The recipient email address does not exist on the mail server. The mailbox may have been deleted or the address is misspelled.
Quick fix
Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.
# Verify email address is correct
# Check for typos in domain and username
# Test with SMTP commands
telnet mail.example.com 25
HELO test
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
# Check MX records
dig MX example.comHow to diagnose Email errors
SMTP errors follow a strict convention: a 4xx code is temporary and will be retried, a 5xx is permanent and will not. Beyond delivery mechanics, most modern email problems are authentication problems (SPF, DKIM and DMARC alignment) which cause silent filtering into spam rather than an explicit bounce, and therefore need to be checked proactively.
If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.
- Read the full bounce message. The enhanced status code (for example
5.7.1) is far more specific than the three-digit code and usually names the exact policy that rejected the message. - Test the SMTP conversation manually with
swaks --to [email protected] --server smtp.example.com --tls. It shows each command and response. - Verify SPF, DKIM and DMARC records with
dig TXT example.com,dig TXT selector._domainkey.example.comanddig TXT _dmarc.example.com. - Check DMARC alignment, not just presence: the domain in the
From:header must align with the SPF or DKIM domain, or DMARC fails even when both pass individually. - Confirm the sending IP is not on a blocklist and has valid reverse DNS. Missing PTR records cause rejection by many large providers.
Tools worth reaching for
swaksdig TXTopenssl s_client -starttls smtppostfix mail logsDMARC aggregate reports
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
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Something missing or wrong?
This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.