Postfix, maildir's, and writing filters

[Da Rock]

Howdy. This may seem simple, but I'm completely green on this: I have a
postfix server with a courier-imap client frontend using maildir's. I'm
using imap for an internal mta, but I need to setup a system which
retains copies of sent emails on the network and not on individual
workstations (which is what happens currently).

I've looked at some of the solutions (bcc and send to a psuedo account
for each user, bcc to the user and filter the incoming mail on this) but
it seems like a very roundabout way of doing things. I've read up on
Postfix, and there is support for custom filters, so:
1. what does it take to write one?
2. how does one copy email from one folder to another in maildirs? Is it
possible?

This idea I have should filter the outgoing mail and copy the messages
to the sent folder as well as retaining its place in the queue.

Any ideas? Maybe a link to some good info? I would like to know how to
do this myself so I can do more in the future so info and pointers would
be great (if you have a script you'd like to share then please show me
how it works :) ).

Cheers