Cascading qmail servers

[Ivailo Tanusheff]

Hi,

I have some problem I can't deal with and maybe because of my not so
good English language skills I can't find in the mail archive.

I have following configuration: a WAN network, with single internet
access point, protected by firewall and a mail server. I also have
several remote offices connected trough a 64K links, every one with it's
virtual mail sub-domain in form xxx.whatever.com. 

                         _office1.whatever.com
<Internet>|{Mail server}<-office2.whatever.com
                         -officen.whatever.com
                         

Because of the bandwidth of the mail traffic I'll need to install some
local mail servers in each office. My question is: how to configure the
main server and local servers in offices, so each mail send to
xxx.whatever.com, after receiving by mail.whatever.com to be forwarded
to the proper mail server. Keep in mind, that there are too many
accounts to make aliases for each. Also I'll need my users to send
e-mails trough their local mail servers in the organization and to
internet. And also I want to keep part of the e-mail accounts on the
main server for the whatever.com domain itself. 

Any help is appreciated,
Best regards,
Ivailo Tanusheff