when retrieving a list of users from elsewhere with emails as a shared field this will detect if they exist in the db already

match_users.php

<?php

$emails = array(
  // fill in emails here or use array from somewhere else
);

foreach ($emails as $mail) {
  $query = "SELECT 1 FROM {users} WHERE mail = :mail";
  $exists = db_query_range($query, 0, 1, array(':mail' => $mail))->fetchField();

}