I am working with a customer that’s running Exchange 2010 on-premises and want to move to Office 365 in a hybrid scenario. The hybrid servers are running Exchange 2010. After implementing a hybrid scenario everything was working fine, but after some time I received calls that free/busy wasn’t working correctly anymore.
When a user with a Mailbox in Exchange online and he wants to schedule a meeting with a Mailbox in Exchange 2010 on-premsises all goes well and the user can see all free/busy information from all users.
When a user with an on-premises Mailbox (on Exchange 2010) wants to schedule a meeting with a Mailbox in Office 365 free/busy information from a Mailbox that’s not Domain joined he cannot check the availability information from Mailbox in Exchange Online as shown in the following screenshot: