2.2 Message Syntax

[RFC2554] section 4 specifies messages that are usable by any SASL mechanism that defines a mechanism name, an optional initial response to be encoded by using base64, and a reply also to be encoded by using base64.

[RFC2222] section 7.2 specifies the mapping of the initial response and the reply to specific GSS-API functions. In the SMTP AUTH Extension for SPNEGO, the GSS-API calls are directed to the SPNEGO Extension, so the calls use the syntax specified in [MS-SPNG].