We are running Exchange onprem in a hybrid environment. All accounts sync to azure ad using azure ad connect. We want the Exchange onprem users to be able to use the same sensitivity labels that we configured in O365 compliance portal. For this to work do we need to setup and configure the rms connector onprem? Also must emails sent from onprem route through exchange online for sensitivity labels to work and be available to onprem users? Currently onprem emails route through Cisco ironports.
Yes, you need RMS connector. Not sure the emails needs to route through EXO (never used it) as the initiating connection point for the service is Azure RMS to on-premises. Check with the third-party and Microsoft official support for clarity.