Configuration issues DNS and fetchmail

[Stew]

I have inherited a "black box" which is the POP server,
firewall etc. This box is fetching mail from out ISP via POP
and our users are POPing to the internal address.

I have installed SUSE 10 and Zimbra and would like to test it prior to putting it into production. I have followed the thread for setting up DNS (DNS in a nutshell) internally and also fetchmail (Fetchmail from other servers (aka account aggregation)).

When I first installed Zimbra I was asked about changing the DNS and the Hostname. The hostname is "testmail" and the internal domain is "smc". Server is testmail.smc. I am able to send email out to the internet and to other internal users using the real email address "username@real_domain.com". I cannot receive email from anyone inside or out.

I would like to be able to set up several test users and fetch mail from Zimbra which in turn gets it from the "black box".

Any help on the proper configuration would be apperciated.