vai con questi headers:

$header="From: $nome $cognome\r\n";
$header.="Return-Path: $email\r\n";
$header.="Reply-To: $email\r\n";
$header.="X-MSMail-Priority: High\r\n";
$header.="X-MimeOLE: Php sender html email by On Air\r\n";
$header.="MIME-Version: 1.0\r\n";
$header.="Content-Type: multipart/alternative;\r\n";
$header.="boundary='==Multipart_Boundary_xc75j85x' \r\n";
$header.="--==Multipart_Boundary_xc75j85x\r\n";
$header.="Content-Type: text/plain; charset='iso-8859-1'\r\n";
$header.="Content-Transfer-Encoding: 7bit\r\n";
$header.="--==Multipart_Boundary_xc75j85x\r\n";
$header.="Content-Type: text/html; charset='iso-8859-1'\r\n";
$header.="Content-Transfer-Encoding: 7bit\r\n";
$header.="--==Multipart_Boundary_xc75j85x--\r\n";