Sometimes, emails can get stuck in your Inbox, Outlook or Sent Items folder and you are not allowed to move or delete them, while Outlook reports an “unknown error”. This article describes various solutions to this problem.
If you are unable to delete all emails from a given Outlook folder (not just certain emails), most likely that folder is locked because it is being used by a 3rd party application (for example, it may be scanned by an anti-virus program). Restarting Windows should fix it.
If you are unable to delete only some emails (while others get deleted just fine), then your Outlook data file might be corrupted. If you are using Outlook 2003 or 2007, you can check it for errors using the Outlook Detect & Repair / Diagnostics tool available from the Outlook Help menu. Unfortunately, Microsoft removed the Inbox Repair tool from Outlook 2010.
If repairing your data file didn’t solve the problem, you can also try one of these solutions:
– Empty your Deleted Items mail folder (right click on it and choose “Empty Folder”), then try again to delete emails;
– Hard delete the selected emails: while the emails are highlighted/selected, keep the Shift key pressed and hit the Delete key;
– Restart Outlook in Safe Mode and try deleting the emails from there. To start the Outlook Safe Mode, simply type outlook.exe /safe in your Start -> Run window or Windows Search panel (for Windows 7 / 8);
– Edit the culprit email (Actions -> Edit Message), save it back and try to delete it;
– If you are using WBA (Outlook Web Access), there is a known bug in Microsoft Exchange Server version 5.5, when the mailbox storage limit was exceeded. To fix it, your Exchange Server administrator must update to the latest Exchange Service Pack.