3.1.5.2 Receiving an SMTP_NTLM_Not_Supported_Response Message

When the client state equals sent_authentication_request, the SMTP client MUST change its internal state to completed_authentication and consider that the authentication has failed. The client can then take any appropriate action. This document does not mandate any specific course of action.

Note This response MUST be interpreted using the rules specified in [RFC2821]. These rules include the use of the three-digit status code to infer whether the failure is permanent or temporary, whether or not to generate non-delivery notifications for messages queued on the client, and so on. As implemented, this will be an exception code: 504, which indicates a permanent error.