I have a .blue email address and it's amazing how many sites still won't accept it. I keep a spare Gmail account for these.
You can do quite well at this, if you're willing to not restrict yourself to regexes and commit to some amount of hackery. One system I worked on used a simple regex (just what is described here IIRC - assert the existence of an @ sign), plus did an MX check on the domain, plus warned (not errored) if the domain was within 1 or 2 Levenshtein distance of any of a list of most common email domains (yahoo, gmail, etc). Statistically it seems like we saved people a lot of grief with this simple filtering.
People accidentally typing their name in the email field, stuff like that. I've done that.
The problems with your .blue is obviously completely unrelated to the "email.contains('@')" check the poster is doing.
I keep a Gmail for the same reason.
I tried to add a .wiki link to a Reddit profile recently and their filters also say that domain is invalid.
That's absurd, there's a .wiki that's almost definitely in the top 20 most visited websites in Korea, if not higher.