3.1.5.3.1 Error from NTLM

Any NTLM authentication error is reported as an authentication failure and the client MUST change its internal state to completed_authentication and consider that the authentication has failed. The client can then take any action it considers appropriate; this document does not mandate any specific course of action.

Typical actions are to attempt other (nonauthentication-related) POP3 commands, or to disconnect the connection.