[Exim] Bizarre problem

Hello,

I have a host, let's call it mail.domain1.com running Exim 4. The Exim
installation is completely the default, with no changes at all to the
configure file.

For users that exist on the box, I can send and receive e-mail. Addresses
are of the form user@???. I sent an e-mail from
me@??? to user@??? and it arrived no problem as
did my replies.

I wanted to try handling mail for another domain, so after setting up the
MX records, altered the local_domains line like so...

domainlist local_domains = @ : domain2.com

domain2.com has priority 10 MX to mail.domain1.com

I restarted exim and sent an e-mail from me@??? to
user@???. It arrived as expected.

If I log into the shell for user on mail.domain1.com and start up Pine
(totally default .pinerc) and reply to the e-mail sent from
me@??? I would expect that the To: address shown would be
me@???. It isn't though. It shows the To: address as
user@???.

If I go into the Pine configuration, set the user domain to domain2.com
and then try to reply to the original e-mail, it gets the correct To:
address of me@???. However the From: address is now
incorrect. Rather than showing user@??? it shows
user@???.

I didn't get this behaviour when sendmail was the MTA so am sure it is
something to do with Exim. I just don't know what.

Can anyone shed any light on this?

Kelv