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3.1.5.1 Receiving an SMTP_NTLM_Supported_Response Message

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When the client state equals sent_authentication_request and on receiving this message, a client MUST generate the first NTLM message by calling the NTLM software. The NTLM software then generates NTLM NEGOTIATE_MESSAGE, as specified in [MS-NLMP]. The client MUST then encapsulate the NTLM message, as defined in section 2.2.3, and send it to the server.

Note The server will send the SMTP_NTLM_Supported_Response message only if the client did not embed an NTLM NEGOTIATE_MESSAGE in the SMTP_AUTH_NTLM_Initiation_Command [initial-response] optional parameter.

The state of the client MUST be changed to received_response.

The command sent by the client determines whether the server response is interpreted as an SMTP_NTLM_Supported_Response or an SMTP_AUTH_NTLM_BLOB_Response. Based on ABNF syntax alone, SMTP_NTLM_Supported_Response and SMTP_AUTH_NTLM_BLOB_Response messages appear identical, making a successful distinction between the two impossible. Therefore, the parser MUST distinguish between these messages as follows:

  • If the client previously sent an SMTP_AUTH_NTLM_Initiation_Command without an [initial-response], the server response MUST be parsed as an SMTP_NTLM_Supported_Response message with a human-readable-string. The human-readable-string SHOULD be ignored by the client, except to facilitate troubleshooting and debugging. This string has no consequence on the operation of the protocol.

  • If the client previously sent an SMTP_AUTH_NTLM_Initiation_Command with an [initial-response], the server response MUST be parsed as an SMTP_AUTH_NTLM_BLOB_Response message with a base64-encoded string. The client MUST NOT ignore the base64-encoded string and it MUST be processed by the NTLM software, as described in this document.

Note Status code 334 is also returned by the SMTP_AUTH_NTLM_BLOB_Response message.