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. firstname.lastname@example.org).
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 email@example.com, Autodiscover returns a Mail-Enabled User with target address firstname.lastname@example.org. Next, Outlook wil try an Autodiscover request for this SMTP address.
The best way to show this is by using the Remote Connectivity Analyzer on http:/www.testexchangeconnectivity.com. In the RCA select the Office 365 tab and check Outlook Autodiscover in the Microsoft Office Outlook Connectivity Tests section.
Enter email address, user account and password, enter the verification code and click Perform Test. In the top section of the screen you’ll see that RCA is able to contact autodiscover.exchangelab.nl and retrieve an XML Autodiscover request for email address email@example.com:
When you expand the Additional Details, more information is shown about the information being returned by the Exchange server:
Clearly visible is the redirectAddr property and the email address firstname.lastname@example.org.
RCA will then try to continue based on this SMTP address. It will fail on several methods, but continue on the HTTP redirect option:
When you expand the Additional Details option under the HTTP redirect method, the autodiscover information is shown for the email@example.com (which is the same as firstname.lastname@example.org email address) user:
So, when moving Mailboxes from Exchange on-premises to Exchange Online in a hybrid scenario, there’s no need to change the autodiscover DNS record from on-premises to online. Even better, as long as there are resources on-premises you should leave the DNS record pointing to autodiscover on-premises.