4
require_once XOOPS_ROOT_PATH . '/kernel/user.php';
$user = new xoopsUser($uid);
$mail = $user->email('n');
or
$member_handler =& xoops_getHandler('member');
$user = $member_handler->getUser($uid);
$mail = $user->email('n');
Second approach is better because it may avoid a sql query if that member was already asked before.
You should use 'n' so you get exactly what is on the database.
If you do not use 'n' than the $mail will become a clickable mailto link.