It all dates back to the Andrew Messaging System at CMU, developed in the 1980's. Originally the format was "<username>+<keyword>+<args>@example.net" where the mail server would interpret the keyword and arguments to route the message in whatever unique way that keyword would dictate (e.g. bob+dist+~/[email protected] would read the file mailinglist in Bob's home directory and deliver the email to addresses listed in it). If the keyword was not recognized, it would just deliver normally. So [email protected] and [email protected] were equivalent, and could be used to filter after the fact if desired.