Mail via PPP

[<martine@hackery.concentric.net>]

I have been trying to set up mailing on my home machine, and while I have
figured out most of it, I still have a few questions.

My understanding of mail in linux with ppp:

You run a mail server on your local box, this allows system messages to be
sent to you via mail.
To get your mail from your ISP, you use fetchmail, which sends the mail
through your mail server.
To send outgoing mail, you send all mail to your ISP's smtp server, so
your computer doesn't have to deal with connecting to faraway targets.

I have most of the above set up and working correctly.  My only problems
have been caused by not having a (static) hostname.  My ISP rejects mail
if the return address isn't set correctly.

I would like my return address on all outgoing mail to be my address at my
ISP.  I believe this is done by modifying the headers on outgoing mail?

Currently, I have the following line at the end of my /etc/exim.conf:
*@concentric.net txs@concentric.net Ffrs

My problem basically is, that I don't understand how it all works :)

If you want to reply to me via email, my real address is
txs@concentric.net.

Any help would be appreciated.