Explanation/Reference:
Explanation:
We need to allow Lync federation with only three specific domains to enable Lync communications with those domains.
In the Lync Online admin center, you can configure options for external communications. You have three options. The default option is turned off completely; no external communications. To enable external communications, you can block specific domains but allow all other domains or as in this question, we can allow only specific domains by selecting the "On only for allowed domains".
With this option selected, we need to list the domains that are allowed. We need to add the three domains to the Blocked Or Allowed Domains list and select the 'Allowed' option for each domain.