Many sites won't accept email addresses with + in them, because many devs have extremely wrongheaded ideas about validation.

I used to have a first.m.last@university.edu address and that one was touch-and-go as well due to the fact that the mailbox had two .'s in it. I actually had to file a support request to get Amazon Student to accept it, even. Nobody from a university with that scheme ever registered before?

For the record, the gold standard for email validation is "send a confirmation link and see if they click it". Don't try and get fancy.

One other trick is that Gmail ignores .'s in addresses entirely. first.last@gmail.com is the same as firstlast or f.irstlast.