In the previous articles I showed you how to implement DirSync, create an Exchange hybrid environment with a migration endpoint and how to migrate Mailboxes from Exchange on-premises to Exchange Online. Not a single word on autodiscover though, and even when autodiscover is pointing to your on-premises Exchange environment, it continues to work for Mailboxes that have been migrated to Exchange Online. This is one of the advantages of an Exchange hybrid scenario.
This is what happens: when you move a Mailbox from Exchange on-premises to Exchange Online, the Mailbox on-premises is converted to a Mail-Enabled User (Remote Mailbox) and a target address is set to Exchange Online (i.e. email@example.com).
When an Outlook client does an Autodiscover request to the Exchange environment it detects the user is a Mail-Enabled User, and that a target address is set. Based on this target address a new Autodiscover request is initiated. So, Outlook does a request for a user called firstname.lastname@example.org, Autodiscover returns a Mail-Enabled User with target address email@example.com. Next, Outlook wil try an Autodiscover request for this SMTP address.