Similar to managing federation, the Public IM providers can be allowed or blocked when configuring an Edge Server. By default, all the included providers are disabled and must be enabled before users can communicate with contacts in these domains.
The following additional options are available when dealing with the Public IM providers:
• Allow Communications Only with Users Verified by This Provider—This is the default setting and it means the Edge Server trusts the Public IM provider’s determination of valid or invalid users trying to send messages to the Lync Server users.
• Allow Communications Only with Users on Recipients’ Contact Lists—This setting limits communication only to users explicitly added to the contact ...