Confused newbie, can I do this?

I've had a linux box set up for a while now using fetchmail to get email
from my isp. I've set up my MUA (evolution) to send mail directly to my
isp's name server. This is working fine.

I'd like however to allow programs to send e-mail through the isp as well.
For example for bug reporting. So I've been investigating how to
configure an MTA to map user names on the linux system to isp names and
send the email through the isp's server. I'd like to use postfix since it
seems easier than sendmail, but having perused lots of docs I'm still
lacking some basic understanding.

I've got a dyndns domain, but I'd rather not expose my MTA to the
internet (virtual hosting?) at least as a first step, so I'd like to
continue to receive incoming internet mail through fetchmail and be able
to send out mail through my MTA to the isp in such a way that replies come
back to the equivalent isp address.

What I've got in mind is something like:

Incoming internet mail: Fetchmail from isp maps mail addresses ->
procmail(SpamAssassin) -> postfix incoming queue.
Outgoing internet mail: application -> postfix which maps mail addresses
-> smtp to isp
Intranet mail via postfix w/o address mapping

The kind of address mapping would be something like:

rick@mydomain.homeip.net -> denatale@ctc.net
deborah@mydomain.homeip.net -> ddenatale@ctc.net
tester@mydomain.homeip.net -> denatale@ctc.net

I'm not looking for detailed instructions, not that I'd mind, just a
general indication of which corners of the documentation are relavant, and
which would be leading me down the garden path.

Any help would be appreciated.