Every SaaS company I’ve worked for has had a compatibility matrix where we say what we support. If we lost customers who were running a highly customized Firefox on Linux, so be it.
Every company decides which customers are worth going after.
And I'd include that as well: if your server rejects emails because of your spam-decisions, you can't claim "we've never received that email". Either you don't use email for any legally-binding communication ever, or spam-filtering is a you-problem, not an everyone-else-problem.
It's not surprising that the strongest protections always happen on the unsubscribe links, but not on the subscribe-links. That just needs to be fined out of existence, just like "you can order with one click, but you need 50 clicks and a three-hour-conversation to cancel".
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.
Every company decides which customers are worth going after.