Like I said before, those are the exact settings I'm using. 

I'm not using my att.net email account, I have several others.  I'm getting the error saying the server is rejecting my other email addresses I'm trying to use even though I've configured the outgoing email as you suggest.  

Error-  The sender address xxx@xxx.com was rejected by the server smtp.att.yahoo.com.

Connection Doctor on the mail client can connect to the outgoing server, but it looks like because I'm not sending from my att.net account, the smtp server is rejecting my send request.