There's a common design flaw on many many websites that require an email address to register; most recently I came across this bug on CNet's download.com site: for some reason they don't accept me when I try to register an email address containing a "+", and they then send me back saying my address is invalid. It isn't!

Screening is a useful way of avoiding spam and/or other unwanted correspondence.

The "+tag" portion of the email address can then be used both for filing mail and for screening it.