AT&T SMTP now enforcing "From" email address
I posted earlier about not being able to send email via SMTP and have now figured out the problem. Starting within the last day or so, they are now checking to see that the "From" address on outgoing email matches your AT&T email address and, if not, are rejecting it. This is probably an anti-spam measure, but can cause problems.
This is a problem for me since I have my own domain so that I can publicly use an address such as "firstname.lastname@example.org" and have it forward to "email@example.com". The advantage is that I can switch ISPs (which I have done a couple times) without having to change my email address in dozens of places around the web, notify contacts, etc. Email addresses are often used as account logins these days, so changing it is a huge honking deal!
I could still use "me @mydomain.com" as the "Reply-to" address, but this can confuse people since they will still see both this and my att.net address and probably not know which is which. So I am going to have to use a different SMTP server to send from my AT&T account in my client email program, which is doable but a hassle.
For what it's worth, Gmail's SMTP servers also do not like this, but they will actually change the From address to match the account used to log onto the server (the reason why that's not a solution for me either).