There is a problem with the latest version of the Mail for Windows 10 App (Version 16005.12827.20200.0) if you are using this for your Gmail account. All mail sent is ending up in your personal Spam folder rather than the Sent mail folder. This makes it look as if the email was never sent although in fact it was. You have to open the sent email in the Spam folder and click that it is Not Spam to allocate it to the Sent Mail folder, extra work and very confusing because it also adds it to the Inbox as well. This only started happening with the latest App Version 16005.12827.20200.0 which was released on May 28th.
I have reported it in the Feedback hub - please upvote! Feedback Hub - Windows Insider
I have reported it in the Feedback hub - please upvote! Feedback Hub - Windows Insider