pfSense

People using e-mail clients to connect to e-mail servers should be using port 587 (not 25). Sometimes this will be through a firewall, so it makes sense to add this to the GUI Firewall Rules, Edit, Destination Port Range drop-down.

Ref: http://mostlygeek.com/tech/smtp-on-port-587/comment-page-1/

Gmail, for instance, uses port 587 for message (e-mail) submission if you are not using the web.
Ref: http://support.google.com/mail/bin/answer.py?hl=en&answer=13287