Log Message:
Update to 2.5.5. From the announcement message:

This is a bugfix release containing a single important fix which could
cause mail to be rejected improperly for domains which advertise a
"discardable" mail policy.  If you are using the \ 
"UseASPDiscard" feature,
this upgrade should be considered mandatory.

The formal release notes entry:

2.5.5           2008/04/25
        Fix bug #SF1947301: Close up a logic problem in "UseASPDiscard"
                handling which could cause false rejections of mail from
                domains advertising "discardable" policies.  Problem noted
                by Doug Kingston.
        LIBDKIM: Another compatibility fix with respect to Sleepycat DB.