I run Outlook 2007 on Windows Vista. I have two accounts in Outlook.
From sometime, I noticed that Outlook was trying to send a message through one of the accounts. The only problem was there were no messages in the Outbox. It kept trying to send these messages. It would try, not succeed, return an error messages saying “Check the address of recipients”.
I did some search and found this useful link.
The problem is usually because of a read receipt stuck in gallows of your Outlook mail folder. It will not display in the Outlook interface. But as soon as you install the program from Windows that is available on the site, you can fix it in a matter of seconds. It will delete the rebel file and all other files can pass through the account easily.