Outlook sends messages on startup but outbox is empty because of stuck read receipt confirmations, Messages stuck in outbox

 I run into two issue with Outlook 16:

1. Outlook says sending x of y messages on startup, but outbox is empty
2. Messages are stuck in outbox

How-I-fixed-it:

I found out, that the lock was caused by read confirmations stuck in the send queue, which did not occur in the outbox.

First of all, I disabled the read confimation sending in the Outlook options (E-Mail -> Read Receipts)

The solution then was to abort the submit of the stucked read confirmations using the Tool MFCMAPI as described here:

Delete a stuck Read Receipt - HowTo-Outlook