Outlook 365 Switch to Exchange 2010 Regedit

https://justinho.com/blog/2018/12/31/Outlook2019OnPremiseExchange.html

https://knowledgebase.cobweb.com/help/outlook-autodiscover-registry-fix

Workaround

1. Run regedit as the affected user on the client.
2. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover (create the key if needed)
3. Add DWORD ExcludeExplicitO365Endpoint and set value to 1

Further Reading

Office 365 Outlook IMAP and SMTP

 https://itservices.usc.edu/office365/emailclients/