impact of Gmail's "promotions" tab on free software communities

[Mat Witts]

It doesn't seem to me to be a great strategy to single out gmail when
Microsoft's hotmail/live/outlook mailservers regularly fail to notify
recipients that they have not delivered mail under the rule that
'Deliverability to Outlook.com is based on your reputation'. Mailserver
admins are then encouraged to sign up for a microsoft account to help
microsoft generate up to date metrics on mailservers. I am sure other
major proprietary systems take similar 'closed-world' assumptions about
transporting email?

https://postmaster.live.com/snds/