It turns out these invisible messages don't live in the outbox at all, in fact they're not even real email messages! The mystery failed emails turned out to be 'read receipts', generated by third parties (the senders) requesting read receipts when sending mail to my users. These failed read receipts don't live in any folder that can be viewed normally, they sort of live in the very root of your personal folders, the 'Outlook Today' page that is seen when you select the name of your personal folder in your folder tree.
How to delete stuck read receipts
- Download this rather cool but of software called 'Outlook Spy' - http://www.outlookspy.com/ it's totally free and blows everything Outlook tries to hide wide open!
- When Outlook Spy is installed open Outlook and click once on the root of your personal folders/ mailbox so you see the 'Outlook Today' page. You'll notice that there's a new toolbar visible in Outlook with all kinds of tempting looking buttons on it.
- Click the IMsgStore button on the Outlook Spy toolbar.
- Outlook Spy will pop up and you need to click the 'Open Root Container' button (top left)
- On the next dialogue box 'GetContents Table' tab (it looks like a button and it on the top row).
- In the left hand pane of this new window you'll be able to see the mystery stuck emails/ read receipts.
- One by one open up each of the messages and on the new window that pops up choose the 'IMsgStore::Abort Submit' button, a message will pop up telling you the message has been deleted, close the window and move onto the next message until they are all gone. Don't be alarmed if you get an error and the message doesn't disappear, you can also fairly brutally just delete all of these messages from the screen that initially shows you them (see point above) but I wouldn't recommend it.
To save your self from users that suffer from 'Red Button Syndrome' it's well worth hiding this toolbar from them by deactivating it in the 'view, toolbars' menu in Outlook.
Thanks go to this site for helping me find a solution to this problem -