The most common cause is the folder still being subscribed to in the IMAP folder list; other causes include retention policies, retention labels, or damage in PST/OST files. Make sure you back up your emails safely. Also, note that the folder will not be recoverable in the future.

1. Delete From Webmail or Unsubscribe from a Folder

Since Outlook can be linked to multiple accounts and syncs many folders, you may sometimes want to delete folders to clear the clutter. In some cases, items inside a folder can block deletion, and checking each item isn’t practical if the folder is large. The solution here is two-fold: first, try deleting the folder from your webmail (in a browser). If that doesn’t work, unsubscribe from the folder in Outlook and then delete it. Follow these steps:

2. Open Outlook in Safe Mode

If the method above doesn’t help, another option is to run Outlook in Safe Mode and try deleting the folder again. Safe Mode starts Outlook without add-ins or custom settings, which often removes whatever is blocking the deletion.

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