http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff637586.aspx
With domain federation, your users can communicate with Lync 2010 users in other companies. Also, your users can communicate with contacts from federated domains by using any of the communication modes that are supported by both companies, such as peer-to-peer instant messaging, audio and video calls, and file transfer.
You can enable domain federation and explicitly recognize certain domains as allowed or blocked, or disable federation so employees cannot communicate with people in other domains. You can choose to:
- Allow federation with all domains except those I block: Allows communication with all domains except those that you specifically select to block.
- Block federation with all domains except those I allow: Blocks communication with all domains except those that you specifically select to allow.
- Disable federation: Prevents communication with all other domains.
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To enable or disable domain federation:
- In the Lync Online Control Panel, click Domain federation. The current domain federation status is displayed.
- Next to the status text, click Edit.
- In the Choose the domain federation setting for all users dialog box, choose a domain federation option, and then click OK.
To add an allowed or blocked domain:
- In the Lync Online Control Panel, click Domain federation.
- Click Add domain. The Domain federation page opens.
- In the Domain name box, type the domain name you want to allow or block, and then click OK. The domain you added is displayed in the federation list, with its domain federation status (allowed or blocked).
To delete an allowed or blocked domain:
- In the Lync Online Control Panel, click Domain federation.
- In the federation list, select the check box next to each domain you want to delete, and then click Remove domain.
- On the confirmation dialog box, click Yes.