Maybe theres option to disable automatic deletion of older emails, i dont know how long they last but it think that it automatically deleted emails older than 1 month, didin't see any option to disable that, maybe you know what i dont :P

Don't have better idea because I am on Windows 8.1 right now.

I use Windows 10 but without any apps, got rid of system apps as well. Last time I used mail app (@ Windows 10 mobile) , it had a problem requiring you to reinstall app but because unfortunately you can't uninstall a system app I had to revert back to 8.1

But this ( http://answers.microsoft.com/en-us/windows/forum/apps_windows_10-outlook_mail/mail-app-not-showing-old-emails/ec21453d-9ecd-46c1-9171-a476449d0a7a?auth=1 ) might help

Are you sure it deletes it? that sounds very odd, is it simply not kept cached but not deleted from the server side? Or is it maybe your email provider deleting them and not your client? I don't see why a mail client would ever delete emails without you asking but it could stop syncing old ones.

Are you sure they are being deleted. If you look at the settings for email accounts in the mail app it only downloads emails for the past month by default

yes they are deleted and happened twice then i was forced to install ms outlook :P

Are you sure you configured as IMAP? If it's somehow using pop and the server is misconfigured any downloaded email in the client will be deleted on the server. Don't see anything else as i've never heard of a client deleting emails server side without user intervention.

How is this possible to use without apps?

How is this possible to use without apps?

This just made absolutely no sense, how is "what" possible? What do you mean by "without apps"?