Dreamhost supports email addresses of the form email@example.com
The mail goes to the same address as firstname.lastname@example.org, but you can filter on the +whatever
It astounds me how many web pages refuse to accept an email address with a + in it as valid. It's valid. Really. I get the mail.
Parsing whether something is a valid email address is hard. (See the O'Reilly Mastering Regular Expressions book for a serious attempt.) If you're not willing to go to those lengths, Don't Try. You'll just make people with unusual-looking but valid email addresses mad.
If you really need the address to be valid, then send mail to it, and make the user do something to prove he received the mail.
If you don't, then trust the user. He knows more about his email address than you do.