Header Loop

Hi,

I have searched the forum and cannot find a solution. Basically I have some clients that use a third party spam filter. The way it works is the mail MX records are pointed to the mail filter MX records and the mail is passed thru our server to the third party server and then delivered back to our server so client can retrieve their mail as per usual.

Now what is happening is that the client is not receiving the emails. I have checked with the third party mail server and they are receiving the email and then passing it back to our server. This is where the problem starts. The email then gets lost or stuck and not delivered to my client and a rejection notice is received by the sender sent by my server. The error message is:
Too many "Received" headers - suspected mail loop

Now I can bypass this by adding the domain name to localdomains in /etc but this is not acceptable.

What am I doing wrong. This was working about 1-2 weeks ago and has been for over 12 months.

Please HELP!!!

Andrew