[Exim] rewrite header or remailer for virtual addresses?

Hi,

I'm running Exim 3.34 (from source) on a Trustix 1.5 box. I've got
virtual domain handling working nicely as outlined in the Exim Spec doc
but now here is the scenario I would like:

1. I receive a message addressed to my virtual account.
2. I reply to the message with the outgoing header containing reply info
for the virtual account. Exim would automatically take care of this
based on received envelope or header info.
3. The original sender receives my reply and can then reply to that with
the virtual address intact.

In other words I would like to keep all correspondence in the virtual
domain if the original message was sent to the virtual address. Is this
a typical thing to do in Exim or do I need to go to a remailer like
Anubis?

Thanks for your help,
Stephen