Mail Problem

I have an issue with sending email via PHP which may be a configuration problem with either PHP, Apache, or possibly a Sendmail, but I don't know which yet. I figured I'd start here first. Here's the situation. I have several webpages that send email to users for various reasons. We have our webserver, an intranet webserver, configured to connect to our smtp server which sits on a different box. The script looks like this: <?PHP $from = "webmaster@swimwebs.com"; $to = "someemail@swimwebs.com"; $subject = "Test"; $msg = "This is a test from my site\n\nTimestamp: " . date("r"); $headers = "From:$from\r\n"; if(!mail($to,$subject,$msg,$headers)) { die("Unable to send"); } ?> This works great when it works. Users receive email as expected and when they hit reply it goes back to the webmaster email address. The problem is if the to email address isn't a valid one. When this happens, the mail server bounces the message back to apache@intranet-server instead of webmaster@swimwebs.com. I've tried adding reply-to to the mail headers and that doesn't work either. We tried sending the same failed message using webmin and that worked great, which is why I believe this to be a PHP or Apache problem. Any ideas? Thanks, Robbert