There is a thread from 2013 here and other places about this common issue except the solution going to Options/Mail/Save messages: Set auto save items to 0 or uncheck is unchecked 0 and sometimes saves messages to draft folder as well as sends them. Not all the time and even if finish message quickly like in 30 seconds I have had it wind up in both draft and sent…..
In theory, this shouldn’t be happening and maybe only me?
I am using Outlook 2016/360 on an IMAP server, Windows 10 pro