If you have hidden, unknown messages that won't send, it's most likely read receipts. They are stored as hidden messages at the root level of your mailbox.

The advantage of using MFCMAPI is that it does not install a program, the exe runs without making changes to your computer and it does not require you to close and restart Outlook.

Download and run MFCMAPI. Use the 32-bit version with Outlook 2007 and earlier or Outlook 2010 32-bit and the 64-bit build with Outlook 2010 64-bit.

1. From the Session menu, choose Logon and Display Store table

Log on to default store

2. Next, go to MDB menu and Open Default Message store.

Open the Default message store

When a secondary account has its own message store and places the outgoing messages in its own Outbox, select that account and right click, choose Open Store.

Tip: you can right click on the account in the list and choose Open Store instead.

3. Right click on Root Container and choose Open Contents Table

Open the root container folder

4. In the Contents table, right click a read receipt and choose Abort Submit. Ok the dialogs.

Right click on the receipt

5. Right click again and choose Delete message. OK the dialog.

Delete the read receipt

Repeat steps 4 and 5 for each read receipt you need to delete.

Note: Outlook Connector (Live/Hotmail/MSN) accounts do not return read receipts and you will not have any "stuck" receipts with these accounts.