When trying to add a new mail box, Postfix Admin rejects some valid email addresses claiming they are invalid (you can find some examples in ). If I correctly understand the code the function in question is check_email in functions.inc.php. The regex checking for validity is too simple to encompass the complex universe of email addresses.
I suggest the adaption of said function so it conforms to RFC 2822. Some approaches for an implementation in PHP can be found in this article .