[exim] How can exim do this?

I am tasked to design, build and maintain a new MTA. The company has
one special requirement, for which I'm investigating the simplest solution:

For each incoming message received, every outgoing "reply" must use the
"To:" address from the incoming received message as the "From:" address
in the outgoing reply.

This is trivial when the incoming has "To: sally@???," Sally
reads and replies, and the outgoing message has "From: sally@???"

It's trickier when the incoming has "To: info@???," the MTA
delivers to an Exchange server, which distributes that message to Sally,
and Sally replies. What is the simplest way for that outgoing message
to use "From: info@????"

Scope is roughly one hundred (100) different domains and, possibly,
thousands of combinations with various username@???.

Ideally, the MTA will handle all of header address processing, whatever
that process might look like. I seek the simplest solution, regardless
how that process looks and compares to the status quo.

Please, advise. Thank you.

~Mike