Log Message:
Update to 2.5.1. Quoting the official announcement message:

This is a bugfix release following 2.5.0, fixing a few minor problems
introduced in that release as well as updating to the latest
Authentication-Results: header draft.

The formal release notes entry:

2.5.1           2008/03/20
        Update for draft-kucherawy-sender-auth-header-14.
        Add "subject" to "should_signhdrs" per RFC4871 \ 
section 5.5.
        Fix bug #SF1911328: Restore proper behaviour of SignHeaders and
                OmitHeaders, broken in the prior release's configuration
                overhaul.  Problem reported by Jason Molzen.
        Fix bug #SF1912332: Fix parameters passed to db->open().  Problem
                reported by Tony Earnshaw.
        Fix bug #SF1912569: Initialize mutexes before entering test mode.
                Patch from Kaspar Brand.
        LIBDKIM: More boundary checking fixes in dkim_canon_selecthdrs().
                Problem noted by Warren Horvath.
        LIBDKIM: Fix bug #SF1820084: Return DKIM_STAT_MULTIDNSREPLY
                if a DNS query returns multiple records.