apostrophe in email name

Filter is_email and sanitize_email, then apply a better check:

namespace Wpse\Email;

add_filter( 'is_email',       __NAMESPACE__ . '\check_mail_address', 10, 2 );
add_filter( 'sanitize_email', __NAMESPACE__ . '\check_mail_address', 10, 2 );

function check_mail_address( $result, $email )
{
    return filter_var( $email, FILTER_VALIDATE_EMAIL );
}

The PHP function is still not RFC 5321 compliant, but better than the WordPress check.