Zimbra pre-install work

[MikeD9000]

I'm preparing to install Zimbra on a spare server that I have, but after reading all of the install files, I'm confused about the DNS. It's a small install with less than 10 accounts.

I have two domains, call them domain1.com and domain2.com that host our mail remotely. They handle POP and SMTP and I would like to keep it that way. I would like to install Zimbra as a web client that can be accessed anywhere behind our firewall and then setup Zimbra accounts that will fetch the mail via POP from domain1.com and domain2.com. I don't have a mail.domain.com setup. Zimbra will not be the IMAP/POP/SMTP servers, I would like to continue to use our remote servers to handle this. Essentially I would like to replace Thunderbird with Zimbra and allow employees outside of the company to VPN in and then login to their Zimbra account to view their mail.

My questions are in this role:

1) Is Zimbra able to do this?
2) If so, how do I go about setting up the DNS correctly?
3) I installed Ubuntu 8.04 LTS on the box and set the IP to a static 192.168.1.251.

Thanks in advance for any help and steering me in the right direction.

[phoenix]

Details for setting up a Splt DNS are in the wiki, there's also a set for Ubuntu if you search for that word.