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No idea! But I'm going to find out :)

So we don’t think we’re explicitly blocking that and have users with that domain.

Could you DM / email me the details and a screen shot? I’ll try and help as best I can.

Have you tried signing up with an @yandex.com email? The signup page just reloads with this message:

1 error prohibited this user from being saved: Email is not from an allowed domain.

You don't need to have a Yandex account for that. Just put in whatever@yandex.com as your email and you get the above message.