Recently I observed this issue during the migration of Exchange 2010 to Exchange 2016. Following are the symptoms noticed not for all but for very few users (less than 1%).
1. Unable to do Deep search on Outlook Address Book (Cached mode), such as with phone number, department etc.
2. Unable to succeed “Test E-mail AutoConfiguration” test from Outlook
3. Availability information were not displaying
4. Outlook returns following error while configuring profile,
"An encrypted connection to your mail server is not available.
Click next to attempt using an unencrypted connection."
And similar other symptoms which occurs due to inadequate autodiscover information.
Suspected Reason and Solution
As you may aware, the mailbox move disconnects the Exchange 2010 mailboxes and connects the Exchange 2016 mailbox during the “Completing” stage if migration/move process. Which means, following stale information are present in the infra,
1. Mailbox Move for each user in the Migration batch
2. Disconnected Mailboxes at Exchange 2010 infra
The issue happens (I assume) because of the incomplete replication of information, and possibly due to the conflicts between connected and disconnected mailboxes. The solution in my case was fairly simple,
1. Remove the “Completed” move request against the user
2. Restart "MSExchangeAutodiscoverAppPool" WebappPool on all Exchange 2016 servers "Restart-WebAppPool MSExchangeAutodiscoverAppPool"
3. Wait for 10 – 20 minutes, and try to test the autodiscover
The above helped me to restore the services back for the user who impacted. Share your experience in the form of comments.