Exim user@domain -> onebox@local -> user@dovecot.local?

[R Ransbottom <vze4zk6s@verizon.net>]

I am trying to set up my first IMAP mailserver.  I am swimming
in unfamiliar waters and would appreciate any pointers.

I am using fetchmail to retrieve mail, exim to send mail, and exim to 
locally deliver mail to a dovecot IMAP server.
Pairnic, our domain provider, reroutes all mail to anyuser@mydomain to
oneuser@myisp.  I want to reroute such mail to anyuser@mailin.private.  
This seems like a problem that someone must have solved already.
Pairnic doesn't add any "X-" header that tells where the mail was
addressed when incoming to them.

I am capable of writing a script to parse the "Received:" headers to
rewrite the address but I don't know where or how to wedge the script
into my mail system.  A oneuser@mydomain forward file seems like it
could work but seems klunky.

Be well,
rir