Please refer to two previous posts:
Windows 10 Mail App–Problems Update
and
Windows 10 Mail App–PROBLEMS
I looks like it was not just me – See https://www.forbes.com/sites/gordonkelly/2020/06/28/google-gmail-serious-problem-deleted-emails-spam/#54bf33f2447c
(Thanks Greg for letting me know).
This is exactly what was happening with me – When forwarding Emails sent to my Gmail and dealt with in the Win 10 Mail App, the sent item “disappears and cannot be found anywhere including in Sent, Drafts, Outbox, Spam, nor on any folder in Gmail on the web (via Chrome Browser).” It just disappears and has obviously been erased from the server.
Gmail in the App is unusable and I deal with emails to that address either in the Chrome Browser and/or in the Outlook Mail program.
Outlook (mail program) is a great program, but it is “overkill” for a simple personal email user like me AND, I don’t like the browser Gmail client at all. I have retained the Win 10 mail App, as it is the default mail client, is simple to set up and operate and works OK with other types of Email (Microsoft, Yahoo, Bigpond and my own Domain) – APART from the occasional formatting issue that I can live with!!.
The obvious alternative is Mozilla Thunderbird and I was about to set up Thunderbird, but I will wait to see if the Win 10 Mail App gets fixed.
In the meantime, I have removed Gmail from the App and will access it via a browser or on my iPad.
A work around found online involves going into Gmail Settings / Filters and Blocked Addresses and creating a new filter to stop emails to your address being put into Spam. DON’T DO THIS. I tried and I don’t even know if it works. I had to immediately delete the filter as spam was ending up in my Inbox.