I have a number of users who all connect to an exchange server in the USA via a VPN connection. They use a mixture of Outlook 2003 and 2007. All are generally able to connect to the VPN and download messages without any problems. 2 of them (one Outlook 2003 and the other 2007) have recently found that their Outlook reports to be "unable to connect to the server" despite the VPN still showing connected, and being able to access other information via the VPN.
If they stop and restart the VPN connection, Outlook the realises it is there again and continues correctly, but after a while it stops recognising it.

Any thoughts?