At work I've introduced mailcheck.js[1] (which I found on HN, BTW.) and we use it in registration form to give hints about mistyped popular polish and international domains. We do also have some validation code, but it's simple and it handles + sign correctly. It's important not only for users, but essencial for developers as well - right now I have 56 unique accounts on the development server using the same e-mail but different things before the + sign. It helps tremendously.

[1] https://github.com/Kicksend/mailcheck