I suspect that there may have been some attempts to fix an issue that isn't being relayed but I don't understand what. This was working up until recently, then their Windows desktop was re-installed and now we have this issue. There's a bunch of history for this user which I'm not privy too and won't be able to find out, at some point there have been application passwords and MFA but these have been deleted / disabled. So there's something in this users configuration that is preventing Outlook from even attempting to connect to Exchange Online. All other users in the same tenant are fine. I've tried from the users system, my own system, several networks and with a new profile in a known working version of Outlook. I tested this with a different account on a different tenant and it worked immediately over this hotspot. The moment I try to login to Outlook for Web an entry appears for this IP (and sign in to OfW is successful). After half an hour or so of sporadic sign-in attempts the AzureAD Sign-Ins page shows no attempts for this IP. To demonstrate this I've attempted a login from a mobile hotspot with a unique IP. If I look at the AzureAD sign in logs there's just no entry for the attempts to log in with Outlook for this account. Whats happening, and this is across multiple machines/versions of Outlook/networks, is that the authentication request doesn't even appear to hit Exchange Online. In Windows Desktop Outlook 365 (up to date) we set up a mail profile as normal and the sign in box pops up, we enter the user and pass and. MS Mail/Andoird Outlook/Outlook for Web are all OK and work as expected. No hybrid, 5 of 6 users are working as expected.ġ of 6 is unable to sign in via Outlook for Windows Desktop.

Exchange online modern authentication