[exim] Manipulating the Date Header on message delivery

Hi all,

This is a long shot but I'll ask anyway ;) - We have a Voicemail system which
stores it's voice messages on our Exim based mail system. We also have voice
customers across 3 different time zones. We know that the Voicemail system
uses the 'Date:' header to tell the user when the message arrived.
Unfortunately, since we don't have the source code to our Voicemail box we
can't make a simple change to the code there to set the 'Date:' header
correctly according to the 'B' party of the incoming voicemail message.

I was wondering if anybody had any suggestions on how we could do this with an
Exim rule/system_filter or otherwise? Basically we would want to look at the
recipient and if it was a recognised pattern match (e.g. vm44xxxx) then alter
the date header accordingly. e.g.

Date: Tue, 5 Oct 2004 17:46:39 +1300

...to...

Date: Tue, 5 Oct 2004 13:46:39 +1300

I realise there are lots of issues here including going back a day etc. but if
anybody has any suggestions on how I might hack this with Exim I'd welcome your
suggestions!

Cheers,
Ray