Basic SMTP reply code · RFC 5321
521 Server does not accept mail
A reply when the connection opens: this host never accepts mail (RFC 7504).
- Class: 5yz · Permanent failure
5yz: a permanent failure. The same request should not be repeated; something must change first, such as the address (RFC 5321 §4.2.1). - Associated enhanced codes
The enhanced codes that usually come with this code, per the registry and the providers:
- Source
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026