Consider the following scenario:
- You are in a Microsoft Skype for Business Server 2015 on-premises and Exchange hybrid environment.
- You implement the Skype for Business Server 2015 Front-End pools with the proxy and without a direct access to the Internet.
- You set up the Skype for Business Server OAuth configuration settings (-ExchangeAutodiscoverUrl-) pointing to your on-premises Exchange CAS server.
- You enable the server-side conversation history feature by using the conversation history configuration settings for Skype for Business.
- You set up an account by creating an Exchange online mailbox.
In this scenario, the user of the account can't use the feature of server-side conversation history to save the conversation history in the Exchange online mailbox. In addition, the on-premises users won't be affected.
The issue occurs because the Exchange CAS server is redirecting the Autodiscover request to Exchange Online directly instead of using the proxy server. As the Exchange on-premises servers can't directly connect to Exchange Online, the connection fails.
In addition, as the on-premises mailboxes are local resource, the Autodiscover process works fine.