When trying to add an extra mailbox to an existing Outlook profile on a computer that is outside the office (connected via VPN) fails with the message: 'The action could not be completed. The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete the action'

Note that webmail from the affected computer is fine, as is a ping to the Exchange server IP.

The problem results from the computer not being able to resolve a path to the Global Catalog on the remote network - simply putting a FQDN entry into the hosts file fixes the problem:

192.168.146.50   es15.contoso.local