Mail.app will not send (att) mail. Web mail and Thunderbird can send.

I'm having the exact same problem on my iMac. I can receive email from imap.mail.yahoo.com, but cannot receive from smtp.mail.yahoo.com. I have not changed these settings in years and as of a few days ago they stopped working. These settings work fine on my iPhone and iPad.

So far I have:

-Deleted accounts and reestablished them

-Deleted the Mail preference file ->com.apple.mail.plist

Finally got this resolved. Yahoo has admitted there is a problem with Mac Mail on iMacs. The temporary fix is the following outgoing settings:

mail.bellsouth.net

Port 25

NO SSL

NO Authentication

They said this is a work around and once the problem is fixed, go back using the normal settings.

Thank you dshuford!! I've been dealing with this for a month. This fix my problem, i can now send email. Thanks