Brad's got the right answer. port 25 is the standard smtp port for receiving email to that computer or domains it acts for.
Bad idea to accept email to any destination on any port without authentication, ie, being an open relay. There are a huge number of bots trying all kinds of ips on port 25 for relaying so ISPs often insist on SSL 587, or if they accept mail on port 25 for any destination, then they'll do something smart with it like authentication or "pop first" or only accept from certain trusted ip ranges (those the isp owns for example), or any one of the many tricks to make it harder for spammers.
