Thank you for your reply.
Address-checked email sent to myself at Outlook 2016 fails to appear in Sent Messages, Sent Items, or Sent Mail (or Outlook.com). It is present in my inbox. A message sent to my wife is gone from my inbox (expected) but does not appear in any of the "Sent" folders.
Also, why do I have three "Sent" folders?
I don't know what to try next