> Not for gmail though; gmail ignores those dots. But [email protected] is normally a different inbox if the local part drops the dot.

My own mail server is configured to use a different character besides the `+` for tags and its notion of "same account" is defined by a mysql query. There's no way to know the interpretation of the local part in advance: you can apply various heuristics if you don't mind annoying people; or you can accept the email and verify it by sending an email.

Similarly, I configured my mail server to use the system "tag.user@domain". So you can't use the same heuristic as gmail, because if you try to do an untagged email, you likely just give the tag, which doesn't correspond to a user.