Not centralizing -- these providers are colluding in a oligopoly. I think there is an argument to be made that these large providers are creating a situation wh

Not centralizing -- these providers are colluding in a oligopoly. I think there is an argument to be made that these large providers are creating a situation where they set up barriers of trust that only allow email from each other to pass. Small independent senders are locked out simply by not being part of the trusted club, even if their email is valid and passes all of the standard hurdles (SPF, DKIM, etc).