Plus sign is [GMail's delimiter for aliases][1], which many people use for filtering and tracking spam.

For example, you want to make sure that your bank's emails are all legitimate so you'll register your email with the bank as `johndoe+[email protected]` and then create a filter that deletes all emails that *don't match* `from:MyBank AND to:johndoe+[email protected]`. Alternatively, when you register with a shady site you can provide your email as `johndoe+[email protected]` and then create a filter to delete everything that is sent to that address.

However, beware that some sites have poorly-written validators on allowed addresses, which block emails with plus sign, which is actually an allowed character per [RFC 5322][2]. Google doesn't allow it simply because it's a reserved character in their system.

 [1]:http://support.google.com/mail/bin/answer.py?hl=en&answer=12096
 [2]:http://tools.ietf.org/html/rfc5322#section-3.2