New Outlook cannot re-add one third-party IMAP mailbox after local removal; another mailbox on same server works

How can Microsoft Support recover an account-specific New Outlook IMAP setup failure after "Remove from this device"?

A standalone third-party IMAP mailbox worked until 18 September 2026, then unexpectedly requested sign-in and can no longer be added. It is not an Exchange/Microsoft 365 shared mailbox. Webmail and Classic Outlook work. A second mailbox on the same IMAP/SMTP service works in New Outlook. The provider confirms no mailbox changes or IMAP/SMTP/authentication impact from server updates.

Reproduction:

1. New Outlook > Add account > affected third-party IMAP address.
2. Attempt setup with provider settings.
3. See "Waiting for your email provider", then "There was a problem. Try again. You may need an app password".
4. After Reset/reinstall/restart, a later error refers to a shared mailbox although this is standalone IMAP (exact wording still needs capturing).
5. Advanced setup also fails.

Expected: successful IMAP addition. Actual: this mailbox cannot be added.

Already attempted without success: app Reset, uninstall/reinstall, Windows restart, Microsoft 365 Quick Repair en-us/lv-lv and Microsoft Support remote troubleshooting. Senior support could not reset cloud state and suggested Feedback Hub.

The account was removed with "Remove from this device" only. It is absent from settings, so "Remove from all devices" is unavailable. Could an account-specific cloud connection/provisioning or classification issue explain this? This is a hypothesis, not a proven root cause.

Please provide a supported recovery/escalation route that preserves provider mailbox messages and a secure way to correlate diagnostics with the existing support case. Account identifiers and case details are omitted publicly and can be supplied privately to authenticated Microsoft Support.

Historical incident versions: Windows 11; New Outlook 1.2026.908.200; client 20260911013.04; WebView2 153.0.4234.32.