Robin syncs with on-premise Exchange through 2 types of connections: inbound and outbound. This diagram shows the full sync cycle.
Inbound
Robin syncs with Exchange through a service account. The service account works like a key. Robin uses it to get updates and to save changes made in Robin back to your calendars in Exchange. These updates are "Inbound" connections to your network that Robin starts.
Outbound
When Robin first connects to a calendar in your Exchange server, it subscribes to notifications about changes to the calendar's schedule. Robin does not have to ask your Exchange server again and again for changes. Instead, Exchange tells Robin when something changes on one of those channels, and Robin updates its copy of the event. This is faster and uses fewer resources.
These updates are "Outbound" from your network, because no outside request starts them. Some network configurations block these connections by default. If changes made in Outlook do not sync back to Robin, make sure your network allows Exchange to tell Robin about changes.