Entonces utiliza la libreria phpmailer:
http://phpmailer.worxware.com/
Código PHP:
Ver originalrequire("class.phpmailer.php");
$mail = new PHPMailer();
$mail->IsSMTP(); // telling the class to use SMTP
$mail->SMTPAuth = true; // turn on SMTP authentication
$mail->Host = "smtp.yahoo.com"; // SMTP server
$mail->Username = "user_name";
$mail->Password = "pass";
// etc..
A ver :)