In a new post to his blog Evert Pot looks at internationalized domain names and where they could cause issues some of the current validation in PHP applications.

Since may 11 TLD's (top-level domain names) have been added. In order for this to work successfully, a lot of applications will have to be fixed. Many email-validation scripts might use [a regular expression] approach [to match most address formats].

Unfortunately, this approach will only continue to work with the normal ASCII-based domain names. He includes a list of several that could break it including ones in Chinese, Korean, Persian, Russian and Arabic. He recommends a very liberal regular expression (one that allows for "punycode" strings) to check the email, find the record and try to find an MX record on that domain.