As a spam mitigation measure, some of the broadband providers are starting to block port 25 for outgoing mail (to keep all those unpatched, zombified windows boxes from flooding the world with more junkmail). Port 25 blocking is old news in some parts of the U.S., but it just hit SBC Customers in California. (I spent a half-hour this evening sorting out why my wife can't send email. Once we had it narrowed down to "hey, I can ping the host, but I can't telnet to port 25" the clue-light went on.)
Here are the details for SBC Global DSL, including workarounds. If you have a different DSL provider, now might be a good time to see what their plans are, so you're not caught by surprise. And if you have a provider you can ssh into, and you aren't tunneling your email through ssh, well, shame on you.