I have configured windows 10 mail application with my google email address.
I sent a mail. I got an acknowledgement email back in reply, but the sent email has disappeared. Its not in sent mail, sent items, or all mail.
I've also checked on mail.google.com directly.
The email had attachments. I've noticed recently if there are attachments its a bit flaky. Previously it got stuck sending forever. To solve that, I deleted the gmail configured account in mail, and re-added. This time it sent in 2 seconds, but as I say there is no trace of the sent email.
I'm running Windows 10 1909 (OS build 18363.836)
Mail app version 16005.12827.20200.0
Even a simple test message sent with the app leaves no trace of the sent message.
If I send from mail.google.com web site, things work as expected