Display custom field value as a mailto: link

well you did all the hard stuff, so you can try this:

 while ( $loop->have_posts() ) : $loop->the_post();
    $mail = get_post_meta($post->ID, 'mail', true);   
    if  ($mail) {
        echo '<a href="https://wordpress.stackexchange.com/questions/60953/mailto:".$mail.'">email me at '.$mail.'</a>';
    } 
 /* ... */

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