Symptoms
In a Microsoft Skype for Business Server 2015 environment, you can't create a Skype meeting on behalf of a specific user in the Outlook for Mac client. In addition, the Outlook on the web (formerly known as Outlook Web App) client and the Outlook for Windows client aren't affected.
Resolution
To fix this issue, install the January 2019 cumulative update 6.0.9319.537 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime.
