500 Command unrecognized

• 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:

  5.5.15.5.25.5.65.7.0

• Source

  RFC 5321

Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026