Checked who is hosting the web site : https://hostingchecker.com
The issue is that the you have a 3rd party Autodiscover on your Website.
You can see it if you click the following link: Link: https://domain.com/Autodiscover/Autodiscover.xml
Exclude root domain in the Outlook Autodiscover journey. Access this link and navigate to Method 9. Head’s up, this solution should be applied per machine in your organization.
Contact your DNS host or Web Host and disable the “AutoDiscover / AutoConfig Support”. Once disabled, access again the link above then you should be expecting “site cannot be reached” error.
Once then, clear outlook cache, restart the machine and setup outlook