I own my own domain in the .us TLD, and I've been using it for my email for the past 20 years. When I applied for an "Electronic Travel Authorisation" (ETA) to enter Great Britain earlier this year the app flat-out rejected my email address as invalid.

Usually, when that sort of thing happens, I just choose to take my business elsewhere or use my time in a different way. However it really ups the ante when a nation-state refuses entry based on that category of technical incompetence.