Because of outgoing spam (either intentional or sent my zombie PCs), most Internet access providers hijack outgoing port 25 SMTP traffic and route it through their system. This way they can catch and block the spammer ASAP. If IAPs don’t do this, they will get put on blocklists and then all their user’s outgoing mail is considered spam or spammy.
There are lots of alternate outgoing SMTP ports, such as port 587, that hosting providers like DreamHost use. My prediction is that eventually the more common ones will also be hijacked by IAPs. I get around all this by SSH tunneling my outgoing mail to a remote server so it appears that it originated on that remote SSH server.
Infinite Ink ~ http://www.ii.com
Deflexion & Reflexion ~ http://deflexion.com