Sure, I can do that. Say a user signs up with
usera@gmail.com. They could sign up again with like
user.a@gmail.com or
usera@googlemail.com. vBulletin will think these are unique addresses because they are different, but they are delivered to the same
usera@gmail.com address. The .'s could be placed anywhere before the "@" (and I am fairly certain you can also use them in conjunction with the @googlemail.com extension).