Your Comcast outgoing server is entered as smpt.comcast.net instead of smtp.comcast.net. Also, the settings should be: STARTTLS security on port 587, authentication = normal password, and User Name = email address.

Also, your att.net and bellsouth POP accounts should be sending on outbound.att.net on port 465, SSL/TLS, but there should be a separate smtp server for each one, with matching User Name and password entries (you already have 2 AT&T smtp servers). Select an account in the left pane of Account Settings, then select the matching smtp in the right pane in Outgoing Server (SMTP).

https://www.att.com/esupport/article.html#!/email-support/KM1010523

Thanks for your help. I think I followed all your instructions correctly but I still get the same error message.

In Account settings, "Server Settings" the Outgoing Server (SMTP) matches up with the correct email address. In the "Outgoing Server (SMTP) Settings" the default server smtp.comcast.net does not have a name or email address in the "User Name" space, it is just blank. Is that correct?

I tried making one of the AT&T - smtp.att.yahoo.com the default server but still same error message. 

Any other ideas before I take it to the repair shop?

Thanks so much, Jay

The Comcast smtp server should have the Comcast email address as the User Name, the authentication should be 'normal password', and the security should be STARTTLS on port 587.