I run into two issue with Outlook 16:
- Outlook says sending x of y messages on startup, but outbox is empty
- 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: