[allegory]

I use a slightly more complex variation of that:

   ^.+@.+\..+$

Works wonders. I think when testing the addresses on a sign up form, we got only 0.5% that we couldn't relay too which was a pretty good hit rate.

[paulnechifor]

It looks to me like you're neglecting email addresses from <http://ai./>. The 'ai' TLD does have an MX record.

[allegory]

Yes; I get told that every time I mention it :-)

[tragomaskhalos]

I would suspect that anyone whose email did not match that regex would have such a miserable time generally getting it rejected as invalid left right and centre that they would just cave in and get a simpler one that did.

[TezzellEnt]

That's pretty good. I learned from Rails Tutorial to use this one:

    /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i

It's worked for me when I needed to use it.

[PinguTS]

Hope you don't have international customers, because http://äöüß.de is a totally legal domain these days.