The email validation regex in eZ Publish, in lib/ezutils/classes/ezmail.php, does not accept apostrophe characters in email addresses, which are legal according to the RFC. From RFC3696:

Without quotes, local-parts may consist of any combination of
alphabetic characters, digits, or any of the special characters

! # $ % & ' * + - / = ? ^ _ ` .

~

As an aside, I've noticed that the ezcMail component accepts ' characters, among others.

Steps to reproduce

1. Try to register an account using an email address containing a "'" character