DNS is fine. Some applications, however, require an public domain on the Internet to have '.' inside... A regex that requires a '.'!
Most applications I've run across that do email validation are way too restrictive. If you have a 4 letter or longer top level domain, many will reject your email address and more exotics like a plus in your email, a percent in your email, etc... will almost certainly be rejected.
One of the most embarrassing of such cases is from Google themselves. For some time (it is fixed now), there was a bug in the job application form. If you used an email with more than two parts to the domain name (email@example.com), it would be flagged as invalid. I've seen other sites make this error too.