A few ISP's now route all traffic on port 25 through their own SMTP servers (Comcast does this I believe). On the upside it allows them to stop virus mail or hijacked computers from sending out spam as all mail goes through their servers. On the downside, you have to deal with the reliability of your ISP's mail server, and sophisticated users can't use their own mail server.

Cheers,

Alex
--
Gossamer Threads Inc.