Changes to the format of the Postfix log data were made sometime between 2.3 and 2.3.1 that break pflogsumm. Specifically, pflogsumm omits the delivered and deferred counts and other important data, making its output non-useful.
Fix: The patch below is the FreeBSD-style unified diff equivalent of the pflogsumm patch included with the postfix 2.3.1 distribution tarball. The following patch is a drop-in replacement for files/patch-pflogsumm:
How-To-Repeat: Run the stock (port-patched) pflogsumm against log data produced by 2.3.1 and see it report 100% rejection and other creative analysis.
Over to maintainer