[Exim] email x-header to mainlog

Hi,

I tried to find solution by myself but I feel manuals
don't cover this issue (also readconf.c doesn't have
keywords?).

Some earlier posts on this list referred to this issue
but final solution was not given (it was about
$message_age to mainlog)?

My problem is that I have email header "x-message-id" in
my mail messages and I need to have that in mainlog. Exims
own mail/queue Id is not sufficient because it changes
from machine to machine and I need to track that message
and its reply.

Is this possible in Exim 4.10 and how (actual config preferred)?
- direct log write (best solution)?
- embedded perl?
- shadow transport with headers only and run filter on it?
- transport filter?

Best regards, Jori