[exim] Alerting users to obsolete email forwarding

I'm looking at how to warn users of an old format of email address.
We used to be in sub-domains, j.knight@??? and now we use
Google Apps for Education on one domain of j.knight@???.

We have exim forwarding email from j.knight@??? to
j.knight@??? and what I'd like to do now is to flag all those
messages so that users can find all the messages that are still using the
old form of address.

The best plan to date is to alter the subject line so instead of
Subject: test message
the user will get
Subject: [Obsolete address j.knight@??? used] test message

There are drawbacks to that plan, especially with anything that does any
kind of digital signature based on the contents of the subject line,
however the users have had 18 months to convert anything to the new address
so hopefully we're only picking up a tiny amount of mail (and a whole lot
of spam)

I was looking at "headers_rewrite" but that seems to be restricted to
addresses, so sysfilter looked like a good option or filtering a transport
through a script to make the change.

I wondered if anyone had done something similar and if so, what did you use?


--
Jonathan Knight
IT Services
Keele University