Follow these steps in classic Outlook for Windows:
- Confirm internet connection
- Open a browser (Edge or Chrome) and browse to
http://www.microsoft.comorhttp://www.bing.com. - If those sites do not load, fix the network first using the Windows network troubleshooter.
- Open a browser (Edge or Chrome) and browse to
- Force a manual send/receive
- In Outlook, go to the View tab.
- Select the Sync button (or Send/Receive All Folders if available).
- Watch the status bar at the bottom for any sync or error messages.
- Check the “Send immediately when connected” setting
- In Outlook, select File > Options.
- Select Advanced.
- In the Send and receive section, make sure Send immediately when connected is enabled.
- Select OK, then try sending again.
- If messages still stay in Outbox
- Leave one test message in the Outbox.
- Use the manual sync again and check whether it moves to Sent Items or shows an error.
- If errors appear, run the Outlook troubleshooters described in the “Issues sending and receiving email” article to detect and repair profile or configuration problems.
If none of the above resolves the issue, use the classic Outlook troubleshooters and, if needed, contact support as described in the references.
References: