We’re in a hybrid deployment with all user mailboxes migrated. The only mailboxes remaining on premise are termed employee mailboxes retained for standard period of time (no delegate access) and system mailboxes used by our ticketing systems using basic authentication via EWS.

I’ve read documentation that says ALL MAILBOXES must be migrated prior to changing Autodiscover, but I’m not sure if that merely means all mailboxes that potentially use Outlook?

Can someone clarify?