Our users are unable to use the Outlook To Do app in the Outlook desktop client.

The hero we needed, it worked on the spot, thank you!

Just an update from my side...

We have discovered that the IP addresses which the To-Do app connects to vary from user to user.
(As far as I can see, Outlook's To-Do app uses the underlying Webview2 Network Service)

For some users, To-Do connects to "known" O365 IP addresses (52.x.x.x and 104.x.x.x), which are listed by MS:
https://learn.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-worldwide
For others (including myself), To-Do connects to "unknown" MS IP addresses (20.x.x.x and 13.x.x.x), which are not listed by MS.
(But RIPE identifies them as owned by MS)

For the first group of users, To-Do works in all contexts.
For the latter, To-Do fails with VPN in split tunnel, as the "unknown" addresses are not covered by the tunnel or firewall policy.

However, these addresses change regularly and are within quite large ranges.
So we asked MS for specific address ranges - and classification of these.

Unfortunately, MS has chosen to close the case with the response that they cannot provide us with this information - but that we can test ourselves with whitelisting and tunneling of the addresses we see used.

Best regards
Martin

Thank you for the update, Andreas.
Even though it breaks our current theory :-)

We might try opening a case with MS - but it seems that people have not had much luck with that so far.

Best regards
Martin

Hi Martin,

In our company, the issue happens even in the central network (after wiping the office cache), with Outlook compatibility option set. We have the Version 2302 (Build 16130.20644) with Semi-Annual Enterprise channel).

Best Regards,

Andreas