Casper can only help you with what’s manageable via .plist file. Unfortunately, Outlook has no preference to suppress error messages like this.
You might actually want to visit Outlook menu --> Preferences... --> Notifications & Sounds and click the button to reset alerts. If user’s have suppressed alerts they may be missing something important to fix this issue. Autodiscover is usually one of these alerts.
If the problem persists and you have access to a laptop experiencing the problem you can quickly localize whether the issue is with your corporate network or Office 365 by traveling outside your network to a local coffee shop or your own home and see if the messages persist. If so, log in to your management portal and you should see options there to open a ticket. Dig deeper and you’ll find a phone number. Microsoft makes this frustratingly difficult to locate.
Hope this helps!