[Originally reported by Eric Singer]
Large messages (apparently the boundary is 64kb) don't validate at other sites such as Yahoo! or Gmail.
Coincidentally, the MILTER_CHUNK_SIZE in the sendmail MTA is right aorund that size.
Using the DKDEBUG trick and the "example" filter that comes in the libmilter source code, the problem becomes visible; a diff of the captured raw message and the data fed to canonicalization differ somewhere around the boundary of the first chunk.