I've tried this contrib, and don't think it solves the problem I'm encountering

The problem is that all mail to local user, from local users situated at a remote site, triggers the RDNS_DYNAMIC rule, which in turn causes the emails to be marked as spam.
For the time being I've reduced the score for that rule to 0.1, which masks the problem, but would like to fix the problem 'properly'.
The other annoyance is that the remote site is actually a fixed IP, assigned from a dynamic range by an ISP!
Any help appreciated
Dan