Users have encountered serious problem linked with google account through Window 10 built in the Mail app. They have reported facing automatic deletion of email from the account or sending them to the spam folder.
As one user described his experience, when he responded to the emails, neither he could found the sent emails in spam, inbox, sent items, or nothing. Moreover, the disappeared email was also not found on the Gmail interface.
The problem has escalated even more and has spread the issue over the Outlook as well. Microsoft has released the note stating that they have realized the issue and is working forward to solving the problems.
Google has confirmed that it is the Microsoft issue rather than on its side. On the other hand, Microsoft has acknowledged the issue of syncing with Gmail and working to solve the issue. As cross-company collaboration is needed to be made so the issue so taking time to get resolved.
At this point in time, it would be best in avoiding the Windows built-in the Mail app and waiting for the patch from Microsoft. And for those who have lost their emails, answer is yet to come from the Google side.