Exchange emails formatting lost

On a system running Exchange Server 5.5 SP3, on Windows NT Server 4, all external inbound and outbound emails that have been formatted as Rich Text are converted to Plain Text. Any internal emails stay formatted correctly. HTML formatted emails (both internal and external) are also formatted correctly.

The other program that is running that may have an influence is Trend Interscan. The exchange component is running on the mail server (scanning all mail attachments) and the other component ison the proxy server (scanning all incoming and outgoing internet traffic).

Any suggestions appreciated.