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You can suppress this message by adding the list of "trusted" servers under the following reg key:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\<xx.0>\Outlook\AutoDiscover\RedirectServers
More details and example can be found in the official documentation: https://learn.microsoft.com/en-us/outlook/troubleshoot/connectivity/how-to-suppress-autodiscover-redirect-warning
If you want to use GPO instead, here's the relevant policy: https://admx.help/?Category=Office2016&Policy=outlk16.Office.Microsoft.Policies.Windows::L_AutoDiscoverRedirectServers