What Constitutes a Valid Email Address?

I thought I knew, but it turns out I didn’t.

While searching through the error logs of a multiple locale project at work, I discovered users using accents in their email address, which were failing the system’s validation techniques.
Looking around on the Google, it turns out most people have made the same mistake I did. The actual set of characters allowed for an email address are far greater than I could have ever imagined.

Check out this article on Haacked about the subject.