Persistent error "The message could not be sent"

A week ago I  mis-typed the email address into  To...  and got the error "The message could not be sent".  Subsequently I typed it correctly, sent the message, and continued using Live Mail -- a lot.

Now, whenever I send an email, I get the error regarding the old mis-typed address.  And eventually another pop-up: "There are unsent messages in the Outbox.   Do you want ...."  although the Outbox is empty.

Yes, I made the mistake originally, but do I have to be reminded forever?   What can I do?  I'm using  LiveMail  Version 2011 (Build 15.4.3538.0513).