Mailcheck looks pretty interesting - I'd been considering adding some sort of client side validation for email addresses for my personal projects, and this looks pretty solid. There are a few cases that it doesn't catch that I wish it would, but it appears it would catch a fair percentage of simple mistakes.

For example, it catches foo@gmail, but it doesn't catch foo@outlook. Of course, gmail is going to be the more common case. Still, it looks good overall and an improvement over no client side validation.

Edit: Actually, I see you can define a set of domains to check against. Very nice.