Can't send IMAP mail on MacBook Pro (though Thunderbird works)

I'm completely unable to send mail with two IMAP accounts on OS X 10.4.5 with my MacBook Pro. Thunderbird works fine so I know there are no network issues. The IMAP access works fine for receiving mail. I can also send and receive messages with my .Mac account, but I stop being able to send them after configuring the IMAP account. I've tried deleting my ~/Library/Mail folder and the relevant Keychain entries, but I can't figure out where Mail stores it's server configuration preferences to delete those too (I've repeatedly deleted them, but it seems to remember past SMTP servers).

I'd greatly appreciate any suggestions.

MacBook Pro Mac OS X (10.4.5) 2.16 GHz, 2 GB RAM, 100 GB 7200 RPM HD