[exim] Mail alias handling

Greetings, list..

As a new user of exim (V4.x for Debian GNU/Linux) I wonder if it is
possible to reply to a mail sent to an alias as that alias, not the
alias-owner.

To be specific -

I have several real users -

user1, user2, etc.,

and a number of alias (virtual) users -

alias1, alias2, etc.

all of which are owned by user1.

I know that it is easy to arrange for mail sent to (say) alias1 to be
routed to the alias-owner, user1.

What I would like to be able to do is reply to such an aliased mail,
from user1's account, but have the message be sent as if from alias1.

I use alias IDs to try to track who among mail recipients is selling my
e-mail address, and want to stop my real mail ID leaking out in the
event of having to reply to a message sent to an alias.

Is this possible?

TIA,
--
Geoff Powell