Try to give us more information about the content of the suspicious message, it should say it is suspicious I believe.
This is the Heuristic checking of the mail scanner at work, you can try lowering the sensitivity in the Internet Mail provider, Heuristics Tab. However, before lowering the sensitivity it would be advisable to find why it is being flagged as suspicious. Treat the disease not the symptoms, find that and put that right, rather than avoid the message (as they say don't shoot the messenger).