Fixing SMTP state tracking.

This fixes the case that an SMTP session has multiple mails sent from
the originator but we miss the server's response (e.g., because we
don't see server side packets at all).
Merge remote-tracking branch 'origin/topic/robin/smtp-fix'

* origin/topic/robin/smtp-fix:
  Fixing SMTP state tracking.

BIT-1203 #merged