I suspect it is not really spam but there is something awry with the headers that makes it look to some spam filters like it *might* be spam. Agasin, we would need to examine the headers to see what doesn't look right about it--and prehaps even the body of the message, though that is far less likely.
Another possible cause: Your server's IP address is on a blacklist (check at
http://www.dnsstuff.com/), which means that there is potentially spam originating from your server. Make sure it is not an open relay, and if you are hosting websites, make sure that any mail scripts are securing (i.e., do not allow additional headers such as CC or BCC).