I ran into this with an NVMO mobile provider.
As of about six months ago, AT&T's web site would not accept email addresses without a three-character TLD. I had to get a customer service person on the phone to manually change my address.
Even .us ??? Pretty sure I used my usual domain (enslaves.us) with them for wireless and california landline and u-verse.
Do you mean it was failing with a >3 character TLD?
Even .us ??? Pretty sure I used my usual domain (enslaves.us) with them for wireless and california landline and u-verse.