[Exim] get md5sum of a file saved to maildir

I am archiving copies of all mails coming in and out of an office system
using a shadow delivery to a local maildir-format directory. I wish to
log all of the main header lines and the filename and md5sum of the file
for insurance purposes.

I cannot find a way of referring to the filename of the file for a
particular message so that I can pipe the header lines and filename to
an external programme, so that it can log the headers and calculate the
md5sum of the contents.

One approach might be to pipe the entire contents of the message to an
external program, and let the external program do the filing on disk
after calculating the md5sum. Is it possible to pipe the message itself
followed by the header fields we need in a sensible way?

--
Rory Campbell-Lange
<rory@???>
<www.campbell-lange.net>