If that 5% is 90% of cost to provide the service, forget it. Nobody is going to do a Herculean task to support a niche user.

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.

Yes, sure, but 5% includes stock firefox, zero modifications, zero plugins.

Might still be a business decision, but it's like saying "we'll drop any emails that indicate a mail client other than apple mail/gmail/outlook".

While not that strict, see how far you get hosting your own email as far as not being rejected or automatically classified as spam

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.