[exim] prepending to the localuser - what would be "the exim…

Greetings All,
I am in the process of creating a new route for handling specially formatted internal mail messages and would like understanding how to set up the exim config. I have read through the docs but I am just not getting the whole philosophy.

In a nutshell, I need to:

- recognize an incoming message is one of the 'special' messages (most easily done by examining the source host. all of these will come from one MTA)
- verify that the localuser is all numeric
- rewrite the localhost portion by pre-pending a string
- forward the re-written message to a specific remote host for delivery

My question is were should I perform each piece of the logic.

e.g.
1) matching the format in a router (and reccomendations on the router)
2) verification of the localuser in the router
3) pre-pending the string via a re-write (really not sure of this step)
4) deliverly using the remote_smtp transport

I am running exim4 with the follow build

/usr/sbin/exim -bV
Exim version 4.66 #1 built 08-Feb-2007 05:43:46
Copyright (c) University of Cambridge 2006
Berkeley DB: Berkeley DB 4.5.20: (December 4, 2006)
Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc TCPwrappers OpenSSL Content_Scanning Old_Demime
Lookups: lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmnz dnsdb dsearch ldap ldapdn ldapm mysql nis nis0 nisplus passwd pgsql sqlite
Authenticators: cram_md5 cyrus_sasl dovecot plaintext spa
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp

much thanks in advance,
ray


--
Raymond P Chudzinski
rpc1.0@???