I'm using outlook (MS 365) with Windows 10 on a MS Surface. Every now and then I'v typed mail (old fashioned pop server, no imap, gmail or so) and after sending the body text was replaced with 3 chinese looking characters. Both at the ones receving the mail ánd in my sent items. Body text nowhere to be found ;(
After some googling I changed automatic coding for outgoing messages (in international options in the advanced part of outlook options) from Western European (windows) into Unicode (UTF-8).
That helped, no more chinese, but sometimes mails are entirely empty!
In now unchecked automatic going entirely, see what happens...