MAR-175: Mail server should be opportunistic in using TLS when delivering mail to remove servers:

- Previously the mail server would only deliver mails over plaintext.
- Deploy a simple SMTP server on both client1/client2
machines. Servers are set-up to require/refuse the STARTTLS over
SMTP.
- Added tests for checking if STARTTLS is used when available for mail
delivery.
- Fixed the wrong configurtion (making sure the TLS security level is
properly set for Postfix).