Prova con questo header:
Codice PHP:
$headers = "MIME-Version: 1.0\n";
$headers .= "Content-type: text/plain; charset=iso-8859-1\n";
$headers .= "X-Priority: 3\n";
$headers .= "X-MSMail-Priority: Normal\n";
$headers .= "X-Mailer: php\n";
$headers .= "From: \"Mittente\" <info@mittente.it>\n";