Adding spam check logging is something we have scheduled for a future 16 minor release, probably a 16.1 minor. Initially it will contain logging for SPF checks, since a lot of debug logging we provide is for SPF checks, but it will grow to cover other spam checks when needed.
So keep an eye out for our next minor release and check the release notes.
Larry Duran
Software Developer
SmarterTools Inc.
www.smartertools.com