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. Choose Edit-> 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. By default the scanpst.exe tool is located in;
C:\Program Files\Common Files\System\MSMAPI\<local ID>
To locate your pst-file;
Make sure you include hidden and system files and folders when searching for pst-file
By default it is located in C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\
You can also locate the file by using
Right click the root folder (probably Outlook Today)-> Properties-> button Advanced-> field Filename