I had to send a diagnostic log to support staff yesterday and the problem arose when I saw the size of the log. 30MB.
So Outlook 2007 valiantly tried to get the mail server to accept it but no chance – mail server timed it out before the message could be dispatched. So delete it from the outbox and that will be the end of it.
Since the message is being sent – Outlook will not let you. Cancel and suspend sending – still nothing. Even tried using a different delivery location i.e another pst file and Outlook clings to the message like a drowning man.
Finally remembered to try going into offline mode (File / Work Offlien) Restart outlook and delete from the outbox.
Then arranged to transfer the log via WINSCP to the support organisations secure server.