I’ve been testing various permutations of blocked email domains and ascertained that if an admin includes an asterisk as a wildcard then all registrations will fail.
To replicate I added *.mail.ru as a blocked domain on a test site. I tried to register from both a gmail account and a .co.uk - both attempts were met with: Something went wrong, perhaps this email is already registered, try the forgot password link
Wildcards aren’t necessary in that field, mail.ru will already block all subdomains *.mail.ru.