SPF spam protection system doesn't work correctly:
1. SPF always resolves messages in 'neutral' or 'pass', never 'fail';
2. Wrong behavior of SPF filter for local rules. Default whitelists is used always.
There are problems into libspf2 library. It returns 'neutral' status by default and processing of spf local rules is corrupted.
The problem has been completely fixed since Parallels Plesk Panel 9.5.3 version. Therefore as possible solution please consider Plesk upgrade possibility.