opening other users mailbox using rpc over http

We have 3 Exchange servers, one is a front-end server and two are backend servers.  We have a large number of remote users and no further exchange servers or VPN solution (VPN solution being developed at present).  The remote users connect to the Exchange servers using RPC over HTTPS and that works fine with no problem.  The problems start when the users try to open another users folder, from  File, Open in Outlook - or open an additional mailbox via the Tools, Email Accounts, change, more settings, Advanced "Open these additional mailboxes" option.

Whenever anyone tries this, they get a message saying the server is not available and/or Outlook should be connected.  In the status bar at the bottom of Outlook it says connected.

Can anyone suggest a way to open additional mailboxes remotely?

Who is Participating?
U cant open additional mailboxes in CATCHE mode.....u need a VPN connection...
or using owa   to access ANOTHER mailbox http://urserver/exchange/username
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