Fastmail has that same technique, and it is very nice. I have run into sites where I sign up for an email where they do not allow the "+" (presumably to prevent you from doing a "foo+spam@example.com"). My only concern with this is it seems like with either way, it would be trivial for a spammer to figure out your real email address (i.e. spam@foo.example.com, use a regular expression to convert it to foo@example.com). How hard would it be to maybe alias foo@spam.example.com and spam@foo.example.com so they both go to foo@example.com? or maybe spam@foo123.example.com also aliases. That way it makes it that much harder to derive the original email address.