When Lync connects to an unknown domain, it needs explicit user consent. A dialog is shown asking the user for confirmation on whether it should continue.
This policy gives administrators the ability to provide trusted domain names. If a domain name is added to this list, Lync will trust that domain and will not show the dialog requesting permission. Multiple domain addresses as comma separated values can be provided.
By setting this policy, Lync will not explicitly trust the default domains specified below. It will exclusively trust the domain specified by the policy.
Not Configured (Default)/Disabled: By default the following domains will be trusted: "lync.com, outlook.com, lync.glbdns.microsoft.com, and microsoftonline.com."
Enabled: The list of domains to be trusted. For example: "contoso.com, contoso.co.in"