Users are not receiving emails

on my site, users are suppose to receive an activation code after resgitration but they are not receivng it.
This is the header i am currently using

$header = array();
    $header[] = "MIME-Version: 1.0";
    $header[] = "From: {$domain}<{$adminemail}>";
    $header[] = "Content-type:text/html; charset=iso-8859-1";
    $header[] = "Content-Transfer-Encoding: 7bit";