I am however doing this by accessing the accounts from gMail (I switched from Outlook on the phone because that started hanging for ever). I can still send emails from the accounts on my smartphone, though this is aggravating for anything lengthy or with attachments that have to be transferred from the PC. Also both Quick Repair and the lengthy Online Repair of Office.
Subsequently I found that this was also happening if I hit the From button (to change which account I am sending it from). However when I try to send the message it hangs for a few seconds, closes and reopens (having lost all trace of the attempted message). I can compose an email and select recipients.
Since the evening of Sunday 19th I have been unable to send emails from Outlook on my PC.