It seems many domestic hosted smtp servers are served as spam, even after an history of legit traffic. Pure IP SMTP or SPF forbid spoofing with proper analysis of headers, namely we would be talking about pown accounts or servers.
But how to verify that? Only gmail people can tell us. And even then, we would have to trust people which interest is to kill alternative SMTP services.
I tried to forward a message from my gmail account to my Outlook/Microsoft-hosted work account, and found out that Microsoft was silently eating messages it (incorrectly) thought were spam, without notifying either the sender or recipient.
Outlook.com still rejects messages from an entire c block if they don't like a particular IP, even when other IPs in that range are not black listed anywhere.
To be fair, things have improved and you now get an undelivered mail notification. Used to be the message would simply disappear without a trace.
Let me guess. These Outlook emails are spam.
I notice nowhere does this article say otherwise.