this problem occurs when the "sent items" file/folder is full. God knows how and why it ditermines it's full even though you have a lot of space on the HD)
you can do one of the following:
delete messges from the sent items folder. it's better to delete all the messages if you don't need them.
you can delete the file that contains the sent items (the file with the dbx format)
if you delete it microsoft outlook will make a new blank copy .
if you want to know where the file is located hold your mouse on the sent items , right click and then properties, you will see the location.
that will do it but if that don't work try deleting messages from the inbox too.