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.

▲ toast0 5 hours ago | parent | next [-]

Even .us ??? Pretty sure I used my usual domain (enslaves.us) with them for wireless and california landline and u-verse.

	▲	Bender 5 hours ago \| parent [-]
		Just a guess but .us does not permit whois privacy and perhaps that may be a factor but I am entirely guessing as all my domains have whois privacy enabled and they would not say why their system rejected my domains.

▲ badc0ffee 5 hours ago | parent | prev [-]

Do you mean it was failing with a >3 character TLD?

	▲	abirch 5 hours ago \| parent [-]
		could be < 3 `.io .co .ai`