When we try to delete a message in Outlook Inbox, Sent Items folder or other subfolders, Outlook refuses to do it and may report an Unknown error. To troubleshoot the issue, we can use the methods described below.

Restart your machine. Some third-party application might lock your Outlook data, which could cause this issue. Unless you know which program causes this, the easiest way to resolve the issue is to restart your machine. Basically, these programs could be Virus Scanner, Anti-spam filters, Synchronization software, etc.

Empty the Deleted Items folder. The Deleted Items folder can hold limited items depending on the version of Outlook. If you have many items in the Deleted Items folder and the number of the items hits the folder limit, this issue happens. To fix it, right click on Deleted Items folder and select Empty Folder, then try deleting a message again.

Hard delete the selected messages. Select the email, hold the Shift key and then click the Delete (X) button or press the Delete key on the keyboard. This is just a workaround to make you delete an email by bypassing the Deleted Items folder.

Delete the email in Outlook safe mode. When we are in Outlook safe mode, COM add-ins and Exchange Client Extensions are not loaded. Additionally, many of the customizations that you have made to Outlook are not loaded. Testing in Outlook safe mode can help to exclude all these factors. To do this, press Windows key + R to open the Run command, type outlook /safe in the box and press Enter.

Edit the email and then delete it. If you are unable to delete only certain emails, we may try this method. Double-click to open the problematic email, click Actions > Edit Message under Message tab. Then, type some spaces and save the change. Now, try deleting it again.

Scan and repair your Outlook Data File. This issue may also happen if your Outlook data file is corrupt. To fix it, use the Inbox Repair Tool (Scanpst.exe) to scan and repair your Outlook data file. To use the Inbox Repair Tool, please follow these steps:

1. Find the Scanpst.exe file from the following location:

(Where XX is the version number, 12 for Outlook 2007; 14 for Outlook 2010; 15 for Outlook 2013)

Outlook on a 32-bit version of Windows:

<disk drive>:\Program Files\Microsoft Office\OfficeXX

Outlook on a 64-bit version of Windows:

<disk drive>\Program Files(x86)\ Microsoft Office\OfficeXX

Outlook 2013 Click-To-Run:

<disk drive>:\Program Files\Microsoft Office 15\root\office15

2. Double click the Scanpst.exe file to open the Inbox Repair Tool.

3. Type the path and the file name of the personal folders (.pst) file or click Browse to locate the file by using the Windows file system.

4. Click Start.

For detailed steps, please have a look at the following KB article:

https://support.microsoft.com/en-us/kb/2534513

If you are using an offline Outlook Data File (.ost), the recommended way is to delete or rename the corrupt .ost file and then restart Outlook to sync a new one. By default, we can find the .ost file from the following path:

<disk drive>:\Users\%username%\AppData\Local\Microsoft\Outlook

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