But not every website out there allows you to enter this as a valid email address.
My earlier hypothesis was that this was on purpose, to make sure you don't use a filter on any email they might send. But these days I'm tending to think it's just a bad regexp on their side.
Even worse, some sites let you enter a plus address initially but that address will not work in some account management pages. I had an instance where I signed up to a pizza place with such an address and I could not unsubscribe or edit my mail preferences because of it.
My earlier hypothesis was that this was on purpose, to make sure you don't use a filter on any email they might send. But these days I'm tending to think it's just a bad regexp on their side.