I would suggest noit rejecting spam with SpamAssassin until you've got working thresholds that satisfy you you're not getting false positives.
And if I did, I might want to create a much more sophisticated system, perhaps adding local rules to SpamAssassin for all blocklists not already weighted/used by SpamAssassin, and commenting out all blockliss in exim.conf.
But that's an awful lot of work and will require a long-term maintenance commitment.
Perhaps a solution for a company big enough to have a person assigned as full-time postmaster.
My opinion.
Jeff