Hey Daniel,
it should default to port 25 for SMTP traffic (unless specified otherwise).
You can check what TCP port your listeners are using in GUI > Network > Listeners > Click into each individual listener.
To locate which listener the email actually matched though (to cross check the port).
You'll need to look into the message tracking log of that instance (of the email), look at which IP Interface which the email matched. Then check on your listener, which listener is using that IP interface and you'll then have the port used for that particular email.
I hope this helps.
Regards,
Matthew