php - mail trouble

Hi.

I have installed a php-script membership site at my server. When someone register a new account, they must enter their email-addy and the script will generate a confirmation-email and send it to the email entered.

This email contain a confirmation-link which redirects the person to the registration page. (It's a double opt-in process to verify the ownership of the email address used to register).

My problem is that the confirmation-emails never reach their target. They are returned to me with this message:
(I have renamed the actual email address and server name from the email below)
-------------------------------------
This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

[email protected]
unrouteable mail domain "msn.com"

------ This is a copy of the message, including all the headers. ------

Return-path:
Received: from nobody by server.name.org with local (Exim 4.52)
id 1EL8NS-0003wn-87
for [email protected]; Fri, 30 Sep 2005 02:03:50 +0200
To: [email protected]
Subject: domainname.biz signup URL
From: "domainname.biz"
Reply-To: [email protected]
X-Mailer: Cash Crusader
X-Priority: 3
Message-Id:
Sender: Nobody
Date: Fri, 30 Sep 2005 02:03:50 +0200

This information was submitted at 09/30/2005 00:03 from IP/PROXY: xx.xxx.xxx.xxx. If you didn't submit your email at that time and that IP address isn't yours, then please send the email with headers to [email protected] and disreguard the signup email. This site operates on a double opt-in procedure which requires you to continue the signup process before you receive any more emails. This will be the last email you will receive from domainname.biz unless someone submits your email address once more. If this happens please contact [email protected] with your email address and it can be blocked so you dont receive any in the furture.

To complete your signup with domainname.biz, please follow this url:

http://domainname.biz/pages/signup.php?CO=dc4933bdREFID&[email protected]

AOL Users
-------------------------------------

I have used the script with my website since the beginning of 2004, and it worked fine until a few weeks ago - then I started getting the confirmation emails in return. (If I forward the email manually using the [email protected] - the mail is delivered without any problems)

The script also use a contact form for support questions - this still works fine as always. I have no problems sending and receiving emails otherwise.

Can anyone please help me out here? I don't know what to do, and I need to solve this problem ASAP.

Thanks

B