io ho risolto cosi:

ho risolto cosi... ho dovuto aggiungere un po di intestazioni per farlo funzionare con hotmail.

<?php
$a="dest@dest.it";

$oggetto="oggetto mail";

$messaggio="messaggio";

$mail="mittente@mitt.it";

$emailto="dest@dest.it";
$header = "MIME-Version: 1.0\r\n";
$header .= "Content-type: text/html; charset=iso-8859-1\r\n";
$header .= "Organization: ".$org_var."\r\n";
$header .= "Content-Transfer-encoding: 8bit\r\n";
$header .= "To: ".$emailto."\r\n";
$header .= "From: ".$org_var." <".$mail.">\r\n";
$header .= "Reply-To: ".$org_var." <".$mail.">\r\n";
$header .= "Message-ID: <".md5(uniqid(time()))."@{$_SERVER['SERVER_NAME']}>\r\n";
$header .= "Return-Path: ".$mail."\r\n";
$header .= "X-Priority: 1\r\n";
$header .= "X-MSmail-Priority: High\r\n";
$header .= "X-Mailer: Microsoft Office Outlook, Build 11.0.5510\r\n"; //hotmail and others dont like PHP mailer.
$header .= "X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1441\r\n";
$header .= "X-Sender: ".$mail."\r\n";
$header .= "X-AntiAbuse: This is a solicited email for - ".$org_var." mailing list.\r\n";
$header .= "X-AntiAbuse: Servername - {$_SERVER['SERVER_NAME']}\r\n";
$header .= "X-AntiAbuse: User - ".$mail."\r\n";

mail($a, $oggetto, $messaggio, $header); ?>


le variabili da modificare sono solo le prime 5.
Ciao