Please note that the advice on this page reduces Outlook's security level and could leave you open to an exploit that takes over Outlook and sends mail on your behalf. We highly recommend you use up-to-date antivirus software that Outlook can detect and leave these settings alone......

More...https://www.slipstick.com/develo...rammatic-access-options/ 

Outlook depends upon the Windows Security Center (WSC) on the operating system to detect the status of the antivirus software on the machine. Since the antivirus status isn’t listed within the WSC on Windows Server Operating Systems, it is unable to obtain this information which results in the message we are seeing within Outlook. You can confirm this if you log into the Windows Server console. In the Action Center, you will not see any antivirus information.

https://support.microsof...us-as-unavailable-this-v 

Allow Programmatic Access to Outlook... 

Check programmatic Access Security in Outlook Trust Center. Antivirus status must be Valid:

related Microsoft topic: A program is trying to send an e-mail message on your behalf" warning in Outlook