Sometimes Outlook gets stuck with the status "Sending Message", even though there's nothing in the outbox. Usually this is caused by read receipts stuck in your account.
To solve this issue, we'll use the MFCMAPI utility. MFCMAPI provides low-level access to Outlook’s data stores, allowing us to delete stuck messages.
First, we’re going to download the latest MFCMAPI version from its GitHub repository.
Next, we’ll extract the downloaded file and run mfcmapi.exe from within the extracted folder.
Make sure Outlook is not open before running MFCMAPI.
Choose: Session -> Logon....
Select your Outlook mail profile.
Double-click the line that has the Default Store column set to True.
Right-click on the top-level folder in the left panel and choose: Open contents table.
When you are using an Exchange account, it is called Root – Mailbox. Otherwise it is called Root Container.
You should now see an item with a subject prefixed with the Read:.
Select the message and choose: Actions -> Submit -> Abort submit....
Make sure the message is still selected, then choose: Actions -> Delete message.
Set the option to Permanent delete passing DELETE_HARD_DELETE (unrecoverable), and press OK.
After these steps, the issue should be solved, so you can close MFCMAPI and open Outlook again.