There I've added it for you. It's nothing elegant but should validate most entries, other bogus entries are filtered and sent straight to /dev/null ;)
Pet peeve: sites telling me to put in a valid email address when I already did.
(You're collecting addresses. You'll validate them by mailing to them. If someone doesn't put it in right, they won't get mailed. That's fair. If they do put it in right, but you reject it, that's not fair.)