What's the state of the art in email validation for Rails?
Don't make this harder than it needs to be. Your feature is non-critical; validation's just a basic sanity step to catch typos. I would do it with a simple regex, and not waste the CPU cycles on anything too complicated:
/\A[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]+\z/That was adapted from http://www.regular-expressions.info/email.html -- which you should read if you really want to know all the tradeoffs. If you want a more correct and much more complicated fully RFC822-compliant regex, that's on that page too. But the thing is this: you don't have to get it totally right.
If the address passes validation, you're going to send an email. If the email fails, you're going to get an error message. At which point you can tell the user "Sorry, your friend didn't receive that, would you like to try again?" or flag it for manual review, or just ignore it, or whatever.
These are the same options you'd have to deal with if the address did pass validation. Because even if your validation is perfect and you acquire absolute proof that the address exists, sending could still fail.
The cost of a false positive on validation is low. The benefit of better validation is also low. Validate generously, and worry about errors when they happen.
With Rails 3.0 you can use a email validation without regexp using the Mail gem.
Here is my implementation (packaged as a gem).
I created a gem for email validation in Rails 3. I'm kinda surprised that Rails doesn't include something like this by default.