They will not let you use their SMTP service unless you register and verify which e-mail addresses are going to use.
I believe I've more or less said before that this is non-sense. And now, I say it again.
I just conducted a test to prove what I already knew but decided to do a new test anyway. My mother uses AT&T DSL. I exported a POP3 account provided by my ISP (which is not AT&T), emailed it to mom, established a remove connection to her computer, imported the email address into Outlook Express on her computer, and then received and sent messages from that account on her computer. This involved no contact whatsoever with AT&T.
Sometime last year, I did this with another AT&T user. In that case, I setup a POP3-enabled gmail account in her email client. No calls to AT&T to "register" or "verify" which account she was going to use.
Think about it. Why would an ISP require some kind of registering and approval procedure to setup some non-ISP POP or IMAP account in an email client? The only valid reason for such a requirement would be to prevent serious cases of email spamming, and they can probably detect that with account monitoring tools. The ISP's role is to provide Internet access, which would include a full range of uses (http, ftp. pop, imap, etc.); their role is not to rigidly police how customers use their Internet access.
I agree with you on one point - AT&T support is usually awful.