[Exim] How to conditionally rewrite envelope addresses?

Hi there,

I'm running Exim 4.30 on the systems in question and have to implement
the following:

I need to conditionally rewrite envelope sender domains, i.e.

if recipient_domain == A then sender_domain = X
if recipient_domain == B then sender_domain = Y

Is there a more elegant, simpler way than running additional Exim
instances (that I pass messages to by means of routers that evaluate the
conditions in question) and that have (unconditional) "rewrite" sections
performing the above changes? I.e.

   router_1:
     condition = condition_1
     transport = transport_1
   router_2:
     condition = condition_2
     transport = transport_2

where "transport_<i>" forwards mail to Exim instance <i> running on port
<p_i>.

I would very much appreciate your help, and yes, this is quite urgent.

Thanks,

Ralf