umm, check the info on your provider's site...usually they list name/IP address pairs of their servers (DNS, POP, SMTP;...)
Unless you really just want to do it the hard way
listen on the box for the port something was sent to:
Okay, sift tru the data to look for traffic on the SMTP port, then do a reverse lookup
You should get one (or more) lines of the servers on that IP address...