As someone working to launch an app, this isn't something that I was terribly concerned with handling before launch, but I now have a question.

Is the best way to handle this to allow the '+' symbol in email addresses, but verify that the underlying email address hasn't already been registered so that one email address still can't create multiple accounts?

So if you sign up with covercash+123@gmail.com, should I check the prefix ("covercash") with the domain ("gmail.com") and make sure there aren't any matches already registered like covercash@gmail.com or covercash+111@gmail.com?