Using Classic Outlook on Windows for personal email, calendar, and contact management
Hi, I'm John!
What exact error message you're encountering when trying to send?
Are you using Outlook Classic (desktop app)?
If so, you can try the "Manual Workaround" provided in this Microsoft article:
Please follow the steps under "Manual Workaround" and then restart Outlook after completing them.
Let me know how it goes. Thanks :)