[exim] rewrite unwanted headers from Outlook client

I've setup Exim to support four virtual domains on a system. I'm using
users with the user.domain format and then forwarding email to them
(i.e. user@??? forwards to user.domain on system [user in
/etc/passwd]). As such, whenever I reply to an email using Outlook, it
seems to be inserting a Reply-to: header (or Sender: header ?) with
the email address as user.domain@???. Subsequently, Exim
produces an envelope with the same email address in it
(user.domain@???). If I use Thunderbird to receive and reply to
the message, this does not happen (I get user@??? for the
envelope address instead).

This might be proper behavior as the user that authenticates to send
the message is indeed user.domain, however, I'd like the Reply-to:,
Sender: and envelope to say, just user@???. From reading
through the documentation, I'm guessing that the method to accomplish
this is to use address rewriting (section 31). I've inserted the
following into the rewrite section of the config file:

user.domain@??? user@??? srfF

however, this does not seem to have any effect. I've also tried
header_rewrite in the local_delivery and remote_smtp area of
transports section with similar non-results.

Is anyone else ever encountered this problem? Am I headed in the right
direction to solve this? Any guidance as to how to proceed would be
greatly appreciated.