[exim] Sending mail through "proxy based" content filter

How I accomplish sending mail through a "Proxy based" content filter?
I have only found configuration examples on how to route remote mail
to a "smarthost", but ALL mail should be routed through the Proxy
provided the mail is acceptable (either acceptable for relaying, or
acceptable for local delivery)

What I want to do, is to have a mail host, where all mail, regardless
of if they are local-local, local-remote or remote-local, should be
delivered to 127.0.0.1:10025
Before that, any SPF or DKIM should be verified, since the Proxy
modifies mail, and any DKIM signatures will get bougus of that.

This is a Proxy server, which talks standard SMTP and does content
filtering and modification.

But then, exim needs to listen on 10026 (the Proxy forwards processed
mails to 127.0.0.1:10026), and then, aliases needs to be expanded
using /etc/aliases, any local mail that comes into this port, needs to
be delivered to dovecot's "deliver" command, and any remote mail needs
to be DKIM signed and then sent outside normally. After the mail
arrives on 10026, it does not matter who sent the mail, as all mails
is already ACLed Before entering the Proxy.

Note here that i cannot DKIM sign mail Before its processed by the
Proxy, because the Proxy may modify content.
I also cannot to /etc/aliases expansion Before processing mails
through the Proxy, since the Proxy uses the un-expanded adresses to
know which content filters that should be applied.


Now to the big problem:
I haven't found any way to make exim treat different ports
differently. Any hits or ideas on how to accomplish this?

Or is there already some configuration option that allows you to
process mail through a Proxy-based content filter "automagically"?