[Exim] transport filters in mail gateway

Hello,

I am operating a firewall with exim on it. Exim accepts mail from the
outside and routes it to the mailserver inside (I've set the route_domains
option to avoid improper relaying.) The mailserver inside forwards all its
outbound traffic to my exim which in turn does its thing. This all works
fine.
I've added a transport filter that renames all attachments with
questionable file extensions (such as .exe or .wsh or .vbs) into something
non-executable.
This again works fine, with the side effect that *all* attachments are
renamed, regardless whether they are coming from the outside ("unsafe") or
are being sent from the inside ("presumably safe").

can I differentiate between what is sent to my inside mailserver (serving
domains x.com and y.org) and what is sent to other domains?

Ben

(bhoc at pentagroup in ch)