MSEXCH:MSExchange error / SMTP mail doesn't arrive from a good domain

[BlueManOp]

I have a Windows 2003 SMTP on the front end to filter out spam mail and forward it to my Exchange 5.5 SMTP box. This setup works fine to filter out SPAM mail with the GFI Mail Essentials 10.1 product. Here is my problem. It seems that some mail coming from a known good mail domain from the Internet or internal relayed mail servers will not come through. Some mail does, other mail gets rejected. This is passing through fine on the front end server (windows 2003 with mail filtering) and passing to the Exchange 5.5 server where it gets dumped.

Example: a plain text message from johnsmith@microsoft.com will come in just fine. Another message with a Web Page / HTML page copied to an email and sent in will fail. It is received, and logged on the front end and boomed from Exchange 5.5 SMTP. Keep in mind that this message was going to the same user that received an email just moments prior from the same person. The NDR returned contains the following message:

Unable to deliver the message due to a recipient problem MSEXCH:MSExchangeIS:

Anyone have this type of problem or know of a fix?

I have rebooted the server, reinstalled SP4 and all the patches released.

If anyone has any good means of turning on diagnostic logging / troublshooting that is useful, please also post.

Any information would be appreciated.