I hadn't changed any settings, but for the last day, I get the following message when attempting to send e-mail from Mozilla Thunderbird 3.0.1:
Sending of message failed.
The message could not be sent because the connection to SMTP server smtp.att.yahoo.com was lost in the middle of the transaction. Try again or contact your network administrator.
Other details: The server is smtp.att.yahoo.com and the port is 465. Authentication is used (SSL/TLS). Secure Authentication is not used. I can send with these settings via Outlook. OS is Windows 7.