Recently we got some support request tickets regarding MAC clients which weren’t able to sign-in into Lync because they were missing the necessary root CA certificates.
Those root CA certificates are required in order to sign-in into Lync rather it’s on the internal or external network.

the following guide was written by a colleague of mine, Lior Gilboa and it explain how to install a root CA certificate on a MAC client:

1. Open the Keychain Access application, you can find it in the Launchpad or find it with Spotlight.
   
2. Locate the certificate file (.CER) you have received from your system administrator and double click it.
   The following message will popup, change the Keychain to “System” and click ‘Add’.
   
3. You will be asked to put in your password to approve the Keychain modification,
   The Keychain Access Insert your password and click “Modify Keychain”.
4. The following popup will appear.
   
5. Expand Trust by clicking the small arrow next to it and verify Always Trust is marked.
   Then click “Always Trust” on the popup window.
   
6. You will be asked to insert your password again, insert it and click “Update Settings”.
7. Now, in the Keychain Access application, navigate to System under Key chains and select Certificates under Category on the left pane.
   Verify that the new certificate appears and marked with a small blue point with a white plus sign in it.
   That’s It.

• Jan 13, 2021 For information about the features, security updates, and non-security updates that are included in a given release of Office for Mac, see Release notes for Office for Mac. For early access to new releases, join the Office Insider program.
• Skype on Mac asks for admin credentials when installing or upgrading Skype. If your Mac asks for admin credentials when installing or upgrading Skype, follow these steps. Download the latest Skype installer (.dmg file) from.
• To update Skype on Windows 7 & 8 from within the app: Sign into Skype. Choose Check for updates manually. Note: If you do not see the Help option in Skype, press the ALT key and the toolbar will appear. To update Skype on Mac from within the app: Sign into Skype. Select Skype from the toolbar. Choose Check for updates.

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