SMTP 550: read the rejection reason before retrying

What does this mean?

550 is a permanent negative reply for the requested action. The full text and enhanced code are essential: an unknown recipient, access restriction or spam-policy rejection can all need different fixes. Repeatedly sending the same rejected message usually does not resolve the cause.

What should I check first?

Keep the entire bounce diagnostic and check the recipient address for spelling mistakes. Separate a 5.1.1-style address problem from policy or authentication wording. Record the sending service, time and message ID, and avoid publishing the recipient’s personal address in a forum.

How can I diagnose the cause?

If the reply names an unknown mailbox, confirm that mailbox exists with the recipient through a suitable alternative channel. If it names sender authentication, inspect SPF, DKIM and DMARC results for the actual source. If it names reputation or content policy, use the provider’s documented review process.

How do I fix it safely?

Correct the specific address or approved sender configuration. Do not purchase random IP lists, forge From addresses or remove every link from messages without evidence. If legitimate mail is blocked by policy, reduce problematic sending practices and ask the provider for the exact rule or remediation steps.

Verify the fix and know when to contact your provider

Save configuration before changes and send one controlled retest after correction. Ask your email provider for a trace if the reason is unclear or its outbound server is rejected. Include the complete redacted diagnostic, timestamps and message ID; never send SMTP credentials or unredacted customer correspondence.

Work through these checks in order

1. Read the complete diagnostic and identify whether it names a nonexistent recipient or a sender-policy problem. Confirm spelling and mailbox existence before altering authentication records.
2. For a policy rejection, retain the actual evaluated domains and provider explanation. Check the relevant authentication path rather than changing MX, which handles incoming routing.
3. Retest once after the specific cause is corrected. If the reason remains unclear, send the provider a redacted trace with the full reply and message ID, not your SMTP password.

Which tool can help?

SMTP error lookup · SPF DNS record checker · DKIM public record checker · DMARC DNS record checker

DNS tools show one resolver’s public answers. Record explainers do not authenticate a message, and calculators do not monitor a server. Use the evidence alongside your provider’s logs.