SMTP 521: server does not accept mail
521Server does not accept mailPermanent: retrying will not help
521 5.3.2 mail.example.com does not accept mail
What it means
RFC 7504 registered 521 as "Server does not accept mail". A server sends it in its greeting, instead of the normal 220 banner, to say that it does not take mail at all. Clients are expected to close the connection with QUIT and not retry this host.
It is often used by "dummy" listeners on hosts that should not receive mail, such as a web server that has an A record but no mail role. RFC 7504 also allows 521 in place of 554 for a connection the server refuses outright. Proofpoint and some other filters return 521 with 5.7.1 when they refuse a connection on reputation grounds, which has its own page.
It differs from 556, which is about the destination domain not accepting mail (for example a null MX), and from 554 at connection time, which is the older, more general "no SMTP service here".
Likely causes
- The recipient domain has no MX, so your server fell back to its A record and reached a web host that answers 521.
- An MX record points at a host that was decommissioned or repurposed and now runs a 521 listener.
- You are sending directly to an address host instead of its MX because of a DNS or routing mistake.
- A filtering service refuses your connection with 521 5.7.1 because of your IP reputation.
How to fix it
- Look up the recipient domain’s MX records and check that the host you connected to is one of them.
- If the domain has no MX and only an A record, the domain owner must publish MX records pointing at their real mail service.
- If the 521 comes with 5.7.1 and a reputation message, follow the provider’s delisting steps instead (see the Proofpoint page).
- As the domain owner, if you do not want mail at all, publish a null MX (`example.com. MX 0 .`) so senders fail fast with 556 instead of probing your web host.
Checks that apply
- MX Lookup Shows whether the domain publishes MX records and which hosts they point at, the first thing to verify for a 521.
- A Record Reveals the A record senders fall back to when no MX exists, often the web server that answers 521.
- SMTP Diagnostics Connects to the host and shows the 521 greeting directly, proving whether it ever accepts mail.
FAQ
- Should my server retry after a 521?
- No. 521 is permanent; RFC 7504 says the client should close the session and treat the host as one that does not accept mail.
- Is 521 a blacklist block?
- Usually not. It normally means the host has no mail role; only when it carries 5.7.1 and reputation wording is it a block.