For anyone having problems on Macintosh - Thunderbird with outgoing smtp using smtp.cox.west.net I have found a solution.
Today suddenly my outgoing mail using the above started failing. In the past, I was never able to use my dreamhost mail.mydomain.com outgoing information…yet suddenly today it is my only choice, as cox said they cannot support the thunderbird client.
I added another outgoing server to thunderbird, server name mail.mydomain.com, port 587, use name and password (my email address,) use secure connection…no.
If any one else has problems, this may help.