Using classic Outlook for Windows in business environments
Hello ! I'm hopeful that I will be able to help you find a satisfactory solution.
If you are using the web browser (Outlook.com), try logging in using a private browser window and see if you can send. If it works, you need to clear the browser cache and cookies.
To open a private browser window, right-click on the browser icon on the taskbar and choose the option for a new private or incognito window.
Thanks
Nelson...