Hi,
Thanks for the heads up.
To summarize, auto-discover doesn’t work with Outlook because of what – because Outlook searches only for .json link and not .xml? Is this what you see on Apache logs – only .json link and nothing else?
If so, try applying this patch, and restarting Webmin.
It can be done with the following commands:
curl https://raw.githubusercontent.com/virtualmin/virtualmin-gpl/master/feature-mail.pl -o /usr/share/webmin/virtual-server/feature-mail.pl
/etc/webmin/restart
… and then disabling and re-enabling the feature on Email Settings > Mail Client Configuration page.
Please come back and let us know if that fixes your problem.