mail() function and spam filters

[n8w]

I have a user registration system .. where users receive an email and have to click on the link to validate their account .. but a lot say they never received the validation email .. I think this is because my mail is getting filtered out by their server's spam filter .. is the anything I could do to prevent this?

Here is what my mail to script looks like

[code]
$subject=$thesubject;
$mail=$email_str;
$maili="ocean@illustrationmundo.com";
$headers="";
$headers .= "X-Sender:  $mail <$mail>\n"; //
$headers .="From: $maili <$maili>\n";
$headers .= "Reply-To: $maili <$maili>\n";
$headers .= "Date: ".date("r")."\n";
$headers .= "Message-ID: <".date("YmdHis")."selman@".$_SERVER['SERVER_NAME'].">\n";
$headers .= "Subject: $subject\n"; // subject write here
$headers .= "Return-Path: $maili <$maili>\n";
$headers .= "Delivered-to: $maili <$maili>\n";
$headers .= "MIME-Version: 1.0\n";
$headers .= "X-Priority: 1\n";
$headers .= "Importance: High\n";
$headers .= "X-MSMail-Priority: High\n";
$headers .= "X-Mailer: SelmanD Mailler With PHP!\n";
/* send mail */
mail($mail, $subject , $message, $headers);[/code]