Plus symbols in email addresses

Despite a plus sign being completely legal in an email address, I still come across sites that can't handle it.

The latest one I have had problems with is Virgin's MyVirginMedia site.
Trying to register with an address that has a plus in it goes through the whole process, and then fails with a generic "Sorry, we couldn't register you at this time" message.

Either a:, perform some sort of validation so I don't waste time filling in the rest of the form, or b: (the preferable option) accept that the plus symbol is valid in email addresses.