Due to laziness / ignorance / some combination of both, many domains that use Amazon can be trivially spoofed by anyone else who can send via Amazon. Last I checked, that was true of outlook.com, too. I think there was a recent article about it...
These companies are so hungry for money, they forget that their products need anything else at all after they get to the "charge customer money for service" part. Companies have been told about issues like these for ages now, yet they all act so surprised when the same ancient problem pops up, time and time again.
And if you want to setup an honest mail server as Random Joe, you will struggle to not get blocked by spam filters while these too-big-to-fail mail senders can get away with being on open relay.
These companies are so hungry for money, they forget that their products need anything else at all after they get to the "charge customer money for service" part. Companies have been told about issues like these for ages now, yet they all act so surprised when the same ancient problem pops up, time and time again.