http://www.rfc-editor.org/rfc/rfc3206.txt
RFC 3206: The SYS and AUTH POP Response Codes, R. Gellens2002-02-01T23:00:00-00:00This memo proposes two response codes: SYS and AUTH, which enable clients to unambiguously determine an optimal response to an authentication failure. In addition, a new capability (AUTH-RESP-CODE) is defined. [STANDARDS-TRACK]