mail() differences from PHP3 to 4?

[Anon]

I just switched over from PHP3 to PHP4, with only one problem. The following code used to work beautifully:

$headers .= "From: Me <[e-mail]>\n";
$headers .= "Reply-To: <[e-mail]>\n";
$headers .= "X-Sender: <[e-mail]>\n";
$headers .= "X-Mailer: PHP\n";
$headers .= "Return-Path: <[e-mail]>\n";
$to = "[e-mail]";
$subject = "Subject 1";
$message = "This is message 1, stored in variables.";
mail($to, $subject, $message, $headers);

Now it doesn't. I tried inputting the actual values, so:

mail("[e-mail]", "Subject 2", "This is message 2, not stored in variables.", "From: Me <[e-mail]>\nReply-To: <[e-mail]>\nX-Sender: <[e-mail]>\nX-Mailer: PHP\nReturn-Path: <[e-mail]>\n");

No change. I get no errors, the mail just never shows up. Any ideas?

TIA,

Steph