Permanent failure — do not retry
SMTP 521
host does not accept mail
SMTP 521 means the host you connected to does not accept mail at all. RFC 7504 defines 521 for this case, so a client knows not to retry. You see it when a domain's MX or A record points at a server that has deliberately switched SMTP off.
How it appears
521 Host does not accept mail
521 5.3.2 example.com does not accept mail
What causes it
- The domain does not receive email, and its server says so on connection.
- The MX record points at a web server or a placeholder host.
- The organisation has retired the mail server without updating DNS.
How to fix it
In order — try the first one first.
- Treat the address as undeliverable. Retrying a 521 will not change the outcome.
- Check the domain's MX records. If there are none, or they point at a non-mail host, no address there can receive mail.
- Contact the person another way if you need to reach them.
Common questions
- What does SMTP 521 mean?
- SMTP 521 means the host you connected to does not accept mail at all. RFC 7504 defines 521 for this case, so a client knows not to retry. You see it when a domain's MX or A record points at a server that has deliberately switched SMTP off.
- Should I retry a 521 error?
- No. 521 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 521?
- Treat the address as undeliverable. Retrying a 521 will not change the outcome.
Last reviewed 2026-09-11