When you set up OAuth authentication between an Exchange Server 2013 hybrid on-premises installation and Office 365, OAuth authentication may fail in a proxy scenario.
If the on-premises Exchange organization can successfully connect to Exchange Online, you may receive the following error:
System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond IP_Address:443 at System.Net.Sockets.Socket.EndConnect(IAsyncResult asyncResult)
If the direction from Exchange Online organization to the on-premises Exchange organization is successful, the Free/Busy information works only from a cloud user to an Exchange on-premises user.
This issue occurs because OAuth authentication does not honor InternetWebProxy. This causes an OAuth token retrieval failure.