This is not true. Regardless of how secure and properly setup your server is: Outlook blocks anybody that does not send enough mail, even if you've never sent

This is not true. Regardless of how secure and properly setup your server is: Outlook blocks anybody that does not send enough mail, even if you've never sent any spam and are on a clean block. They're happy to let their users to send you spam, but ironically they still block you when you you try to report it to abuse. The good thing is they block you, so the email bounced and you know it wasn't delivered. Gmail classify messages to a user who has never communicated with you before as spam. This is silent, so you never really know if an email to a Gmail box has been filtered out as spam or not. Their abuse inbox accepts messages but I'm not sure they do anything with it. Basically, email has been hijacked by two companies.