Authentication and connection drops with Outlook accounts (User is authenticated but not connected)

The "user is authenticated but not connected" issue is usually due to temporary Microsoft server issues. The server is not allowing the imap connection. The app usually successfully reconnects again a few hours once MS resolve their issue.

Do you use a VPN to connect?

Thanks for the reply!

I do use a VPN on my smartphone, but it is set up with split tunneling (per-app proxy), and Thunderbird is excluded from it — meaning it connects directly without the VPN.

Also, I doubt this is purely a temporary Microsoft server issue, because the authentication failure and the "User is authenticated but not connected" notification happen multiple times every single day.