Even while it sometimes appears as though Outlook should be able to send an email message, there are sometimes problems, such as outlook emails that get caught in the outbox, that make this impossible.
Fortunately, you may resolve this issue on your own using the troubleshooting techniques listed below.
For optimal results, complete each step one at a time until your problem is solved or you've discovered the approach that suits you the most.

Deactivate automatic updates

You could not be receiving emails from Outlook due of your automatic updates.
When updates are implemented, the Outlook could abruptly quit, stopping email sending and receiving.

Go to File > Options > Advanced to disable automatic updates.
Remove the checkmark from the box next to Check for updates under the Automatic Updates section.

Remove older versions of Outlook

Uninstalling any prior versions of Outlook is one thing you might try if you're having issues with the programme.
This is due to the fact that newer versions and older versions may clash and create issues.

To achieve this, select Outlook from the list of installed programmes by going to Control Panel > Programs and Features.

Uninstall it by doing a right-click on it.

After completing the instructions to remove Outlook,

Switch off your computer.

Check to see whether Outlook can be opened once more.

Resolve if your email is stuck in outlook: https://www.gethumanhelp.com/emails-stuck-in-outlooks-outbox-try-these-steps/