[qmf] Make SMTP AUTH PLAIN more robust.

SMTP authentication using AUTH PLAIN supports two ways to send authentication information to the server,
seems that some servers don't support anymore auth in two steps:

c: AUTH PLAIN
S: 334
C: <UserName and password encoded in base64>
S: 235 2.7.0 Authentication successful

Auth in one step introduced by this commit, reduces communication with the server

c: AUTH PLAIN <UserName and password encoded in base64>
S: 235 2.7.0 Authentication successful