Here's a really simple one...
Check "use regular expression" under question validation and then enter this in the regular expression field:
^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$
This will allow the following formats:joe@aol.com | joe@wrox.co.uk | joe@domain.info and disallow the following formats: a@b | not an email | joe@@.
If you're looking for anything more specific...here's a good reference:
http://regexlib.com/Search.aspx?k=email
Hope that helps! Heather