Your test email address mego_spam20@ukr.net works OK for registration at my site.
blacklist of banned domains
Blocked Email Addresses (Enter one email per line) at UM Settings -> Access -> Other
I figured out what was the reason. I already had a user with this email. It is very strange that the plugin is not able to display that the user is already in the database. Can this edit be made?
Also thanks for the explanation with the domain blacklist, worked.
You can look at this UM guide about validating email addresses.
https://docs.ultimatemember.com/article/1697-email-validation-in-registration