Although I agree with the meta-point, there's still value in doing some basic validation. If there's no @ sign, or the domain name looks like it was mistyped (gmali, yahhoo), then a better user experience for your user is to present them with a warning and ask them to retype their email. You may not want it to be a blocking warning (that is you can still submit the form), but some validation can be a huge value add to users.
Still, sending an email beats regex validation any day for determining it's a real, working address.
Yeah, skip all that too. Who says yahhoo.com isn't a valid domain?
For example, I regularly receive email for an individual who has a nearly identical email address to my own, but at ymail.com instead of gmail.com. Any system that tries to guess at domain misspellings is going to catch ymail.com and think "Ah ha, they meant to type gmail.com, I'll correct that for them!" Viola, their email is sent to me. Again, this is not a theory, it happens to this poor guy all the time.
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.
Still, sending an email beats regex validation any day for determining it's a real, working address.