`AUTH GSSAPI` && `AUTH GS2-KRB5`

[dequbed]

Is your feature request related to a problem? Please describe.

I'm currently looking at what vSMTP would need to provide for us to be able to switch over to it.

Describe the solution you'd like

Hmm… Thinking about is this is actually kind of my job. ¯\_(ツ)_/¯

Oh well, GSSAPI auth support would be nice, we use Kerberos a lot