[rlonn]

Nope, there is no restriction for plus addresses. I validate email addresses and specifically allow + and . in the part before the @ sign. [e-mail] and [e-mail] will be considered two different users.

Have to add also that the "plus feature" isn't a Gmail-specific invention but a standardized way of referring to the same mailbox using different addresses. But some email providers don't support this feature, and many input validators out on the Internet don't either.