[Exim] Adding text to the Subject line

Folks,

I am working for a company that has, over the last year,
accuired a number of subsidiaries, as well as changed its
name. We use exim 3.35, on Debian woody.

All mail is either delivered locally, for users to
POP3/IMAP, or re-routed via /etc/aliases.

Earlier, domain name used to be old.com. After our name
change, we became new.com. There was minimal change to our
setup, as I simply asked users to change their From/Reply
address in their clients to @new.com, and added it to
local_domains. Ditto for susid1.com, subsid2.com, etc.

I now want to start cleaning up, more cleanliness sake; I am
in favour of accepting old.com for ever in principle.

What I need is a way to notify users who recieve mail at the
old.com address that they should contact the sender, and ask
him to use the new address. I think the easiest way may be
to add [OLD.COM] to the subject line when accepting mail, if
condition matches, and asking users to handle these cases
manually with their correspondents.

I am using a system filter (thanks, Douglas Stephens).
Anything I can add to it?

I know an MTA should not be fooling with mail text, but this
seems the least intrusive way.

--
Sanjeev