Problems with VPN access to Outlook and Remote Desktop

Archived from groups: microsoft.public.windowsxp.work_remotely (More info?)

I have intermittent problems accessing Outlook and Remote Desktop on our
network via VPN. I have not tracked down all the details, but it seems to be
as follows:

a) The VPN connection is completely reliable. Once connected, the network
is visible, network drives can be seen and all machines can be 'pinged'

b) From some external locations, it is also possible to make a reliable
connection from Outlook to Exchange Server. When this is possible, it is
also possible to access my office PC via Remote Desktop

c) From other locations, neither Outlook nor Remote Desktop will establish a
connection. However, Outlook Web Access works perfectly in these cases.

I'd appreciate any suggestions as to where to look