When trying to delete a message it could be that Outlook refuses this and shows an error message. Most common error messages are;

"An unknown error has occurred."

and

"Messaging interface has caused an unknown error."

There are 3 easy methods which you can use to solve this for most situations.

Note:
If you have a message stuck in your Outbox see; Message stuck in Outbox.
If you get the error “The Folder is full” see; Folder is full.

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Method 1

Clear the Deleted Items folder and try to delete the message again.

Method 2

Edit the message;

1. Open the message by double clicking on it.
2. Place the message in edit mode;
   • Outlook 2003 and previous
     Choose Edit-> Edit message…
   • Outlook 2007 and Outlook 2010
     Actions-> Edit Message
3. Type or remove some characters (I usually type 3 spaces).
4. Save and close the message.
5. Try to delete the message again.

Method 3

Run scanpst.exe against your Personal Storage (pst-) file. The location of scanpst.exe depends on your version of Outlook and Windows. For an overview of the possible locations see; Inbox Repair Tool (scanpst.exe) or use OutlookTools (free) to locate it for you.

If you do not know the location of your pst-file, you can use one of the following methods to locate it;

• Make sure you include hidden and system files and folders when searching for pst-file
• Default locations are;
  • C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\
  • C:\Users\%username%\AppData\Local\Microsoft\Outlook
  • \My Documents\Outlook Files
• You can also locate the file in Outlook by using;
  • Right click the root folder (probably Outlook Today)-> Properties-> button Advanced-> field Filename
  • File-> Data File Management… (Outlook 2007 and previous)
  • File-> Account Settings-> Accounts Settings…-> tab Data Files (Outlook 2010)