If one tries to add a new user with the email address john's.email@example.com piwik returns The email doesn't have a valid format. even though the address is perfectly valid.
A var_dump($emailAddress) in this line shows that the all special characters get converted to HTML entities.

• john's.email@example.com => john's.email@example.com
• "very.unusual.@.unusual.com"@example.com => "very.unusual.@.unusual.com"@example.com
• !#$%&'*+-/=?^_{|}~@example.com => !#$%&'*+-/=?^_{|}~@example.com