I don’t understand the “automatic” here-yes, reputation takes time to build, but if you run your own mail server with SPF/DKIM/DMARC set up correctly why is the default posture “block it” before there’s any reputation?

Just like other cases, I won’t accept that it’s “just lazy” on the part of big tech companies. They clearly know how to adjust their internal view/reputation of a domain once it starts being used for “misbehaviour” and spam such that they start blocking it.

Thus they could clearly start by not doing so-and, maybe, they’re “really touchy” about domains with no initial “internal score” such that if a new domain pops up and starts spamming people they catch it fast. Its not necessary to break open Internet protocols, though, unless they want the breakage.