Edit: AT&T (BellSouth) appears to have started blocking some messages. Yahoo seems to have stopped blocking.

For some reason, Yahoo has flagged ot.com for "unusual" e-mail activity and is deferring all messages sent from our mail server. I'm getting a lot of bounced e-mails from users subscribed to various forum topics. The topic notification system is working fine; it's Yahoo that's sitting on our e-mails and not passing them on to you.

The only solution I can suggest is to change your e-mail address to something else until Yahoo stops blocking our mail. Just a warning: changing your e-mail address will cause the forum to automatically log you out and send you a new password.