smtp settings are correct. Why can't I send email with AT&T yahoo?

Edited on 12/17/09
I changed my password on ATT/Yahoo and in Thunderbird. I can download messages but can't send. Before I made the change, I had no problems receiving or sending. When sending, I get the following message: "The message could not be sent because connecting to SMTP server smtp.att.yahoo.com failed. Verify your SMTP server setting is correct." I've read all the knowledge base q&A. I have confirmed with ATT that my settings incoming and outgoing are correct. I had this problem before and ATT was able to resolve it...but apparently they can't (or won't)now. I can't even pay for a resolution. Can anyone help? I like Thunderbird and don't want to change unless I absolutely have to. Does anyone know why ATT won't support Thunderbird or Mozilla?
I apparently don't know what I'm doing since I only received 1 response and referred to Canada. I'm in Florida using ATT/Bellsouth.net
 
sad I’m disgusted
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