Hello there, just realized that I can't find my older mails in Windows 10 default mail service(the one with calender). Cant find my older Emails in the Windows 10 mail service. Note that if you find.ost files, the data in them is not generally recoverable (without spending money, at least) Cant find my older Emails in the Windows 10 mail service. That said, there's no harm in doing a thorough search on your drive for.pst files.
Unless you deliberately copied/moved emails to a.pst file, then all (undeleted) emails should still be on the mail server of your ISP. Home Where are my older emails in Windows 10 Mail
