mail() not working with headers

[solitario]

Hello all.
Once I put in the following headers, mail() stopped working. When I take them out, it works. Any clues? Thanks.

$headers='';
    $headers.='From: '.$sending_name.' <'.$sending_email.'>'."\n";
    $headers.='To: '.$array[0]['first_name'].' '.$array[0]['last_name'].' <'.$array[0]['txt_username'].'>'."\n";
    $headers.='Reply-To: '.$sending_name.' <'.$sending_email.'>'."\n";
    $headers.='Date: '.date("r")."\n";
    $headers.='Return-Path: '.$sending_name.' <'.$sending_email.'>'."\n";
    $headers.='MIME-Version: 1.0'."\n";
    $headers.='Content-type: text/html;charset=UTF-8'."\n";