The following change was introduced in 0.4.1:
LIBDK: In dk_eom(), return DK_STAT_SYNTAX if
dk_skipbody is set.
Failing to do so means a message with a signature header
below which there is no sender header will report a bogus
success status. Problem noted by Lennert Buytenhek.
As a side effect this makes it report a syntax error for
messages which are not signed at all, e.g. domain key
signature is not present in the message.
Allowing those messages thru w/o reporting syntax error
like it was before or reporting 'no signature data' would
be a much better way to handle this.