I prefer to do email suggestion rather than validation - The only validation I have is that it should contain an @. And then use something like http://getmailch

I prefer to do email suggestion rather than validation - The only validation I have is that it should contain an @. And then use something like http://getmailcheck.org/ http://getmailcheck.org/ to suggest spelling corrections for common domains. We saw a massive reduction in bad emails just by implementing this without having to annoy users with incorrectly implemented validation rules.