At firewall>lan create a rule like this:
pass, protocol tcp, source lan subnet, destination any, port 25, logging checked, gateway default.
Make sure this rule applies before other rules allowing port 25 out like the default lan to any rule. You'll now see green pass icons in your systemlogs at status>systemlogs, firewall tab. If you want to see more than only the last few items set up a remote syslogserver so you can browse through the past few days/weeks or whatever is needed.