You can verify this by using the Fiddler or TCPView tools and track the attempts to connect to the and servers. Somewhere in the fall of 2016, an update was released for Outlook 2016 that enabled a mandatory check of the Office 365 cloud connection point.
If the computer with a problem Outlook is not joined to the Active Directory domain, then on the contrary, you should try to switc from NTLM authentication to Basic. At the same time, you need to check the authentication type used for the IIS site on the Exchange server. If your Outlook is configured to access the Exchange mailbox using Outlook Anywhere (MAPI over HTTP), verify that NTLM authentication is used.
