Setting up a hybrid configuration between Office 365 and on-premise Exchange may seem straight forward, but there are a lot of pitfalls to tackle. One of the more frequent issues is a problem with free/busy information not being visible, or the inability to migrate mailboxes from/to Exchange Online.

Additionally, you may have received this error during the initial configuration of the hybrid setup:

Hybrid Configuration Wizard
Office 365 was unable to communicate with your on-premises Autodiscover endpoint. This is typically due to incorrect DNS or firewall configuration. The Office 365 tenant is currently configured to use the following URL for Autodiscover queries from the Office 365 tena  ...