I have just migrated one of 3 domains from Office 365 Tenant A to Office 365 Tenant B. I can't get Outlook 2016 clients to connect to the mailboxes on the new tenant at all. I've run the Online Support and Recovery Assistant and this confirms there is an issue with Autodiscover but doesn't expand on what the issue is. Bearing in mind the external dns records remained unchanged and have been verified by the new 365 tenant, I don't see how autodiscover can be wrong. I've tried setting up Outlook on a test PC that has never connected to the old tenant and that fails too so I don't think it's cached dns. Anyone have any ideas what might be going on please?