Cannot delete email in Outbox

Put Classic Outlook into Work Offline mode before deleting the message; this stops Outlook from actively transmitting and locking it.

Remove the stuck message

1. In Classic Outlook, select Send/Receive → Work Offline.
2. Check the status bar and wait until it shows Working Offline.
3. Open Outbox, select the stuck message, and press Delete.
   • To preserve it, drag it to Drafts instead.
   • If it contains a large attachment, open the draft and remove the attachment before attempting to send it again.
4. If Outlook still says it is transmitting the message:
   • Close Outlook and wait a few moments.
   • If it will not close, press Ctrl+Alt+Delete → Task Manager.
   • Select Microsoft Outlook or outlook.exe, then choose End task.
   • Reopen Outlook while still offline, return to Outbox, and delete or move the message.
5. Select Send/Receive → Work Offline again to reconnect. The status bar should no longer show Working Offline.
6. Select Send/Receive All Folders or press F9, then confirm that new mail appears on the desktop and iPhone.

These are Microsoft’s documented steps for releasing a message that Outlook reports as currently being transmitted.^1^

If sending or receiving remains blocked

After the message has been removed:

• Send a small test message without an attachment.
• If Outlook remains disconnected or the test message stays queued, start it in Safe Mode:
  1. Close Outlook.
  2. Press Windows+R.
  3. Enter outlook.exe /safe and press Enter.
  4. Test sending again.
• If sending works in Safe Mode, an add-in is likely interfering. Review File → Options → Add-ins → Manage: COM Add-ins → Go, and disable add-ins for testing one at a time.
• If Safe Mode does not help, use Microsoft’s Classic Outlook troubleshooters. If those do not resolve the issue, the documented next step is Microsoft support.^2^

Do not remove the email account merely to clear one Outbox message; removing an account also removes its locally cached content from Outlook.

References

1. Outlook email stuck | Microsoft Support
2. Issues sending and receiving email | Microsoft Support