Gmail users have discovered that Microsoft’s Windows 10 built-in Mail client is deleting their emails and/or sending them to spam automatically. Google has confirmed this is a Microsoft problem rather than Gmail which means users will have to wait for a Windows 10 update rather than a Google server-side tweak.
There is a Temporary workaround that can be done within Gmail
Gmail.com > Settings > Filters and blocked addresses > Create a new filter
In ‘From’ field enter your email address
Click ‘Create filter’
On the next page, select ‘Never send it to Spam’
Click ‘Create filter’ to save
This workaround creates two sent items in Windows Mail but it does stop Gmail emails being lost or deleted which is the most important thing until Microsoft comes up with a fix.