Following all instructions given by att for continued use of email (which up
until today had worked flawlessly for years), I now am unable to send email
using Outlook Express. Never before had at&t require authentication, but it
appears they do so now, though I'm not sure when this went into effect.

I simply use the DSL service to send and receive email, but I don't use the
account that was assigned, as I have had an account on a friend's server for
probably 10 years or more.

Now, when I try to send, I get an error message indicating messages are
rejected by the server, with the problem apparently centering around
"requiring a valid sender domain."

I didn't see this in a thread here yet, but this is baffling.