I doubt we'll ever turn away a customer by preventing registration of a new account sharing the prefix to a plus sign in their email address with an existing customer.
RFC 822:
The local-part of an addr-spec in a mailbox specification (i.e., the host's name for the mailbox) is understood to be whatever the receiving mail protocol server allows.
I repeat, this has absolutely nothing to do with the mailbox address, where we send mail.
It might make them bad netizens and you may not like it. But the spec doesn't compel any behavior. It's just a way to communicate technical ideas and ideals.
You're under no obligation to accept names that are hard (as in painful) for you (except if you're the police, I suppose. Ironically)
You can't break user expectations and mental models by pointing to the spec as justification. The spec exists to serve users, not the other way around.
In the absence of being able to count on specs, I guess the user should expect a race?
Most of them can't specifically point to the problem, their impression is just that your services don't work properly. They're right.
Assuming they don't just fail to register and move on ...
It's like deciding to not allow quoting and with this whitespace. Sure ":"@example.com is a legal mail address (surprised?) but nothing good will come from allowing it.
Are you suggesting I shouldn't be allowed an account with you if the person who beat me to my preferred email address also beat me to registering with you?
So at best you can special case for those "major hosts" and not apply such treatment to any other domain.
The RFCs for email don't say "uhh dude whatever, just check what gmail does and maybe hotmail too lol". You are playing fast and loose with these things.