SMTP is an email protocol used to push messages and attachments from server to server. Many technologies have been used to secure SMTP over the years, but the best technologies available today use SSL (version 3) or TLS to secure the entire SMTP connection.
SMTP typically uses TCP port 25 to move unsecured traffic and often uses TCP port 465 to move secured traffic. Use of alternate ports with SMTP is extremely common to reduce connections from spammers who try to exploit servers listening on the most common ports.
BEST PRACTICES: SMTP should always be secured using SSL (version 3) or TLS. If your file transfer deployment uses email notifications, then email sent through SMTP should either always be accepted within a few seconds or should be automatically queued up in the file transfer application in case of delay. If SMTP services are unreliable and no queue mechanism exists in your file transfer solution, then a standalone mail server relay should be implemented to ensure that timed-out notifications do not cause unnecessary delay or failure of file transfer activities.