gmail supports having +stuff in your email address too. it's fairly easy to set up filters to put stuff into folders based on the email it got sent to.

That's different from what I described above.

You don't use a '+' at all in the fastmail email addresses you give out.

For example, say with gmail you have [email protected], with fastmail you could use that if you wanted, but you can also use [email protected]

If fastmail receives mail on that address, it converts it for you as if it had been sent to [email protected] instead.

This happens entirely on the fly so you can make 'proper looking' emails without a '+'