Send legitimate e-mails to people you know on those services. Ask them to mark those e-mails as trusted / not spam. Make sure they reply to at least a few. I

Send legitimate e-mails to people you know on those services. Ask them to mark those e-mails as trusted / not spam. Make sure they reply to at least a few. I've been running my own e-mail server for years. I have DKIM, SPF, PTR, and everything else set up and my e-mail domains and IP addresses are many years old. Every so often, Gmail still decides to spam-bin me for a week or so.