Here is a problem that plagues many versions of Microsoft Outlook. You have no emails in the outbox, but every time you use Outlook, it is trying to an email. You may also get a send error. The underlying problem is Outlook had trouble sending a read receipt. Which can cause other read receipts to never send. Emails in the outbox may or may go through as well. In my experience, some did send and some did not.

The problem goes back to so many versions of Outlook, that Microsoft has provided a tool and instructions to fix the problem.

I would also recommend you use the scanpst tool after this, and then compact the PST data file.

Update: Sometimes the Microsoft tool does not work. In that case, you will have to create a new Outlook PST file. You will have to do this with the Mail option in the Control Panel. The only way to properly do that is to add a second instance of the same email address while making sure it uses a new Outlook PST file, then migrate all emails to the second instance within the Outlook program, then close Outlook when done, then go back to the Mail in the Control Panel and remove the first instance of the email address.