When a service preset won't do, turn to "Other."
In the screen that appears tap on Add Mail Account. In the New Account sheet that appears enter your name, the email address for this old account, and the new password. Tap Next and because iOS is aware of these old addresses your account should verify rather than be rejected with an error. Tap Save and your account's set up.
Reluctant though I may be to lend AT&T a shred of sympathy for this mess I will say that transitioning a bunch of old domains such as sbcglobal.net, prodigy.net, and any of a number of bell.nets to a single domain was probably challenging and perhaps this is really the best it can do.
But, honestly, if this is the best it can do you're better off abandoning the old address and using something more up-to-date — a Gmail, Yahoo, or iCloud address, for example.
Yes, it's tedious to shift over to a new address. You can make it a bit less so by replying to incoming messages with the new address as well as sending any new messages from that account. If you like, add some boilerplate text to the bottom of your message indicating that you're using a new address. After awhile you'll find that the majority of people you correspond with will use your new address, at which point — after ensuring that merchant accounts you maintain also use that address — you can drop the old one for good and all.
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