Sorry, I was mistaken, the email search use Begins With. So if you search for:
johndoe
it will return any emails where the username is johndoe, for example:
johndoe@yahoo.com
johndoe@gmail.com
if you wish to change that, you can edit the admin_pom/users_results.php page, and change:
$WADbSearch1->addComparison("UserEmail","".((isset($_GET["S_UserEmail"]))?$_GET["S_UserEmail"]:"") ."","AND","Begins With",0);
to:
$WADbSearch1->addComparison("UserEmail","".((isset($_GET["S_UserEmail"]))?$_GET["S_UserEmail"]:"") ."","AND","Includes",0);