BMF uses detects the string "\nFrom " to check for a new message.
Unfortunately, this is often tricked mid message by new sentences
starting with From on a new line.
The attached patch modifies the new message detection in
bvec_loadmsg (filt.c), requring the detection both of a from line, and
a Message-ID line.
Down-sides? Haven't debugged closely, but will probably miss chunks
of the header when scanning whole mbox's.