I do not quite understand what you want.
In my experience was to configure fetchmail, officemailserver, kerio, sendmail and distribution of local boxes.
Provider by the way was a shared mailbox for all, the sorter then laid out in separate boxes and copied to the Director.
As for the local server, then configure a forwarder in LAN. It will be clubbing too big letters, internal correspondence and forwarding to the upstream server.
Mailforward articles on the Internet is enough for your system https://www.zimbra.com/desktop7/help/en_US/Userspe...