I guess any user has experienced this before: you are opening your inbox and the number of unread documents shows anything but the truth.
There is a very easy but (for some users) inconvenient way to bring the unread marks on line with the reality in selecting Edit -> Unread Marks -> Mark all read.
However, some people prefer not having all documents marked unread since they are using this feature to highlight special messages or tasks they do have to action. In this case you might want to try to select Edit -> Unread Marks -> Scan unread … . Notes might tell you that views are not properly build and ask you if you want them to rebuild. Select ‘yes’. Now, open the mail file and press CTRL + SHIFT + F9 which will rebuild all the views and folders.