Login to Webmail
Have the user login to their webmail (https://login.microsoftonline.com).. If they are able to access their mailbox in webmail, then their credentials are good and they have a mailbox license assigned. This is about all that can be checked on the server side.
Check Additional Mailboxes
Does the user have an additional mailbox in their Outlook profile that is not migrated? If the user is migrated, they can have other migrated mailboxes as additional mailboxes in Outlook. But if they had previously or attempt to add an additional mailbox that isn’t migrated, Outlook may freeze and hang when trying to connect to these mailboxes. Users who are migrated and need to connect to mailboxes that aren’t migrated, will need to do so via OWA (I try to migrate groups of people with the shared mailboxes they access at the same time, but this isn’t always possible when users who access the mailboxes are spanned across multiple departments.)
Recreate the Outlook Profile
Completely remove the Outlook profile and recreate it.
Although if this Reg key is added and then Office is later reinstalled on the machine, I have seen it cause problems. Following a re-install of Office, you may need to remove it and/or remove it and re-add it.
Try the First 2 Steps in this Article:
Switch to Online Mode
Uncheck “Use Cached Exchange Mode” in Outlook profile settings. Note: Unchecking this will put Outlook in Online Mode, and a copy of the user’s mailbox will not be stored locally. If the Internet goes down, this user would not have a local copy of their mailbox to work from until connectivity is restored.
Try Starting Outlook in Safe Mode
Start- Outlook.exe /safe. If it opens and connects in Safe Mode, try turning off all Outlook Add-ins and open Outlook again in regular mode. If works okay, add needed add-ins back in one by one.
Microsoft Diagnostic Tool
This is Microsoft’s diagnostic tool for troubleshooting Outlook problems when connecting to O365. I put this program at \\share\software\o365_migration. It can be time consuming though and isn’t always helpful.
Microsoft Support Ticket
I can open a ticket with Microsoft. We have support for this and Microsoft will help. They can work with a technician at a person’s desk or if you are unable to do it, and authorize it, Microsoft Support can call a user directly and walk them through the troubleshooting steps to fix Outlook.