There is starting to be a few articles on the
problem, but nothing official from Microsoft. It looks like a recent update to the Windows 10 Mail program (not Outlook) has broken something with Gmail accounts. Messages sent from Win 10 Mail seem to disappear from the Sent folder, and can't really be found anywhere. I have had some luck finding most of them in the Spam folder on mail.google.com.
This problem has been driving me nuts for weeks, I have a Gmail account for my business and have been struggling to ensure that my correspondence with clients is actually being sent. Figured I would give the heads up for anyone else who might be seeing weird behavior.