Happens in Gmail and even in Outlook. I have an institution mail ID (from outlook paid office 365 account) that marks my mails from my personal mail ID as spam. My friends who are using an Outlook mail account face the same issue. I got a report for my institution mail ID and the reverse DNS is happening for that ID but it is not happening for the personal mail ID. And it is not a mistake. Outlook is at fault here. The reverse DNS is not happening.