Using New Outlook on Windows for professional communication and productivity
I think I have a couple of work-arounds - at least they worked for me. Instead of opening the email, just click it once and it will open in the view panel rather than in a separate window. From here click 'Send' and it goes.
Another work-around is to open the email fully, try to send it, then close the window. You then answer yes to save to a different folder and you will find the email in your INBOX - ready to send.