There are a number of ports used in relation to email. SMTP/SMTP over SSL use 25/465 respectively. But pop3 uses 110, and pop3 over ssl uses 995. If you're running imap ver. 3 you'll need port 220 open, and imap over ssl uses 993.
And a number of server programs let you reassign port numbers, so you can use services on different ports. Port 25 is what you'll normally need for smtp, but if you need pop3 or imap you'll have additional ports you need to worry about.