Recently, Mail.app began having problems connecting to the various smtp servers I use. At first some worked, while others didn't but then all failed completely after a few days. Took me a while to figure out that SBC has started blocking port 25 - the port used to reach these types of servers. Except for its own, of course.
The irony is that even though SBC spams me as much as anyone, they didn't bother to announce this change - or perhaps Mail.app read it as junk and disappeared the email. Anyway, it turns out that there IS a way around it. You can opt out of the blocking:
"To request removal of the filter, use the Abuse of Service form at http://help.sbcglobal.net/servabuse.php" to contact our Abuse team and choose the "Opt-Out Port 25" under Abuse Type. Enter your account Member ID (also called your SBCIS PPPOE Login ID or User ID) to opt-out. If you have multiple SBCIS accounts you want to opt-out, please submit a separate form for each Member ID. Do not enter your email addresses from the other mail hosting provider. You should expect that it will take SBCIS 12-24 hours to process your request."
I suspect this is an attempt to block all the worm-infested PC zombies from sending email to all the contacts in every victim's address book, but I find it inconvenient, and opted out.
The full article ( you must be an sbc subscriber to read it) is here