1.6 Applicability Statement
The SMTP AUTH Extension for SPNEGO is used when implementing an SMTP client and SMTP server that negotiate a mutually supported authentication mechanism using SPNEGO.
Since this extension and the SASL to GSS-API wrapper defined in [RFC4752] use the same SASL mechanism name, this extension is not applicable to clients in environments with servers that implement the Kerberos V5 ("GSSAPI") SASL Mechanism specified in [RFC4752]. A client in such an environment will use the SPNEGO Extension specified in [MS-SPNG] when the server advertises "GSSAPI", and authentication will fail.
However, the SMTP AUTH Extension for SPNEGO can be used by servers that support Kerberos with clients that use either this extension or the Kerberos V5 ("GSSAPI") SASL Mechanism. When the server advertises "GSSAPI", the former will use the SPNEGO Extension and the latter will use Kerberos directly, and both can succeed.