[Exim] Setting a dynamic From: in Autoreply transport

In an autoreply transport, you can specify what to use as the "From"
header in the generated reply.

If undefined, Exim uses the local_part, domain and system gecos field as
the "From". It can also be specified with "from = $local_part@$domain".
Or, you can specify some global address, "from =
autreply-daemon@???".

But, is there a way you can set this dynamically for virtual domains and
aliases?

For example, a message comes in for "webmaster@???", which is
an alias for "dm4031@???". Their autoreply should say "From:
webmaster@???". But, obviously the local_part Exim sees is
"dm4031".

Can Exim be told to look in the "file = autoreply.msg" for a "From"
header, or somehow use the original RCPT? A lookup?

Rich Sandberg
richs@???