As Thomas said, you could block the domain of your external contacts. But that will also block all communication with them, you will not be able to search for them, chat and meeting with them.
If you want to show your presence only for people in your contact list, you could run the following command:
Get-CsPrivacyConfiguration | Set-CsPrivacyConfiguration -EnablePrivacyMode $True
And also, you could vote on this idea in UserVoice. Some new features would be added to the services based on customers' feedback in the future, and your good ideas will be very helpful for them to improve the service.
Best regards,
Lou
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