We have a big update available for Outlook 2016 this month. This update is for MSI version and takes Outlook to version 16.0.4366.1000. Click to Run users should be at version 16.0.6568.2036.
In addition to updating support for Add-ins (from the Office Store) to API version 1.3, this update fixes the following issues:
When you add a user as a contact in online mode by using the MAPI over HTTP transport protocol, the new contact isn’t created and no error message is displayed. This fix requires either Exchange Server 2013 CU12 (KB3108023) or Exchange Server 2016 CU1 (KB3134844) installed on the Exchange backend as well.
When you connect to an Exchange server, you receive a prompt that lets you connect to the server if the security certificate is mismatched. To fix this issue, you also need to add a registry key. See KB3114926 for more information.
Synchronization of groups fails. (The new Office groups, not contact groups)
Users with online archives may experience connectivity issues, including the Inbox folder doesn't sync, your mailbox is renamed as Archive or an archive mailbox is renamed as your mailbox, and you may see a duplicate view of your mailbox instead of the archive mailbox. If you change the protocol between the MAPI over HTTP and RPC, the primary mailbox isn't displayed.
A number of MAPI over HTTP issues are resolved, including problems with Outlook crashing, email attachments are corrupted during the download, public folders won't open and display the following error message: eCannot display the folder. Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing.
Profile creation fails for hidden global address list (GAL) users.
You may see incorrect search results in shared mailboxes in Outlook 2016.
When Outlook 2016 connects to both an on-premises Exchange Server and Modern-Authentication-enabled Exchange Online, the HTTP 500 error occurs, and the Free/Busy information isn't displayed. This issue occurs if the AlwaysUseMSOAuthForWebServices registry key is set. See KB3114976 for more information.
You can't select multiple attachments of an email message in Outlook 2016 by pressing SHIFT.
Full details are available at April 5, 2016, update for Outlook 2016 (KB3114972)