[exim] Replacing "localhost.localdomain" in Message-id

Hello all!

I finally figured out how to strip the Message-id string out of each
outgoing message, but right now it's being replaced with
"localhost.localdomain" (which isn't too bad actually).

Ideally, I'd like to have the host name that's listed in the Message-id
header change to match the domain listed in the sender's email
address.

For example, If jsmith@??? was sending a message through, the
hostname in the Message-id header would change to 'example.com' and if
foo@??? was to send one through, it'd get changed to 'bar.com'. The
reason for this is that I've got some friends' domains hosted and it
would be better if their emails didn't have anyone else's domain listed
in that header. If nothing else, maybe just have it apply what's listed
in /etc/mailname (Debian system).

Thanx in advance!

/Hans