I ran into this with an NVMO mobile provider. They did not like my personal email domains (assorted .net and .org) so I nagged their customer support until they manually added it. Their marketing team happily emails my personal domains once added. Some day this will probably cause a problem but my goal is to eventually get rid of my cell phone either way.
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.
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.
