The .NET Framework's email address validation has a pretty ... comprehensive ... regular expression [1].
[1] http://referencesource.microsoft.com/#System.ComponentModel....
[1] http://referencesource.microsoft.com/#System.ComponentModel....
I would hate to be the programmer who had to debug that regex.
Behold: http://www.ex-parrot.com/pdw/Mail-RFC822-Address.html