I don't see a problem with using a regex to validate email, unless you get false negatives. Rejecting anything that could be a valid e-mail address is bad and will cause frustration for someone at some point.
I don't think allowing validly formed e-mail addresses that don't have corresponding accounts (or even domains - consider a web app working in offline mode) is a bad thing, especially when taken with point 1, as surely the point of client side validation is a heads up to the user that the data is wrong, rather than actual validation?
I don't see a problem with using a regex to validate email, unless you get false negatives. Rejecting anything that could be a valid e-mail address is bad and will cause frustration for someone at some point.
I don't think allowing validly formed e-mail addresses that don't have corresponding accounts (or even domains - consider a web app working in offline mode) is a bad thing, especially when taken with point 1, as surely the point of client side validation is a heads up to the user that the data is wrong, rather than actual validation?