Problem receiving notification emails to mails sent with php

Hi there, I'm sending emails to the subscribers of my site using mail() with a fourth parameter to specify the From: and Reply-To: headers. They seem to be sent correctly and the users may reply to them and I get the replies in the mailbox identified by these headers. The problem is some email addresses I'm sending email to are incorrect, and I should be receiving the email notification saying that the email could not be delivered. But I'm not receiving these messages in my mailbox. Any ideas why this could be so? Thanks a lot for any hints. I'm sending mails with: mail($to, $subject, $body, $from); where $from = "<webmaster@mysite.com>\nReply-To: webmaster@mysite.com\nX-Mailer: PHP/" . phpversion(); I've also tried using \r\n instead of \n