I have logged on here to find an answer to the same question.
Did you find out how to do it in the end ? Here is a synopsis of my request for info....
The Exchange 5.5 environment :-
One Exchange Organisation with multiple sites. Each office is a site with it's own Exchange Server.
There is an Enterprise Exchange server which links the sites using X400 connectors. The mailbox properties can all be viewed from the
Enterprise server, but the actual databases are of course on each individual office Exchange server.
At present, the Head Office has a Site Server (IIS) with OWA Exchange component, publishing mailboxes for the Head office users on a web site.
The desire is to be able to publish pages for other remote office users from the same site server at Head Office. The remote Office user mailbox databases are on the server in the remote office
The following paragraph was taken from Technet
The Exchange Server used for crawling, the Outlook Web Access server, and the Exchange Server that is connected to the Outlook Web Access server do not have to be the same computer. However, both Exchange Servers must be in
the same organisation, and must share the Public folders. Specifying different computers for the Exchange Server in the Microsoft Exchange configuration properties and the Exchange Server connected to the OWA server
allows you to access messages from one location and display them from another.
However, it appears that the OWA component of Exchange will only allow the configuration of one Exchange server.
Is it possible to publish the mailboxes of more than one site (ie more than one office Exchange server) from the Head office server box ?