Outlook Locks up when laptop not attached to network

[merk]

I have a colleague with a laptop - Outlook works fine when connected to the network. However, when he is not connected to our network (incidentally, Internet access occurs through the LAN), Outlook locks up when he tries to open emails containing attachments.

He logs on with a local user account in both scenarios (he obviously does not connect to the network).

Any help is appreciated.