A number of our clients are experiencing a problem with Outlook 2010 and Windows 7.

Our client machines are running Outlook in cached mode. On some occasions, they try to send a message but the message never reaches its destination, nor does it appear in the sent items folder. Instead the items are going to the Outbox, but can never be retrieved as the message headers to not appear in the list.

In the navigation pane a digit appears next to the "Outbox" folder, but there are no messages to show.

Deleting the OST file removes the digit, but the unsent mail is also lost and the error reoccurs later down the track. This is not acceptable as important mail often goes missing. Our system administrators report that the messages never reach the mail server and the error must be on the client side.

Scanning the OST file for errors and running a hard drive check does not recover this "phantom" unsent mail.