Some sites now seem to check whether an address is valid from some database or heuristic, because a random email with a valid domain is still rejected.
There is definitely a phenomenon of checking for prohibited email domains. However, absent extra-functional motivations like user data collection, smart developers don't bother with heuristics for email addresses much beyond the presence of an '@' character.
If you want to know if a purported email address is deliverable, try to deliver email to it.
