Are you a troll? Why would he lie about this? I’ve been on both sides of this, I self-hosted my email for over a decade, then just this year made the switch to proton. This guy isn’t lying, if you set up your DNS, DMARC, SPF, etc, properly, you don’t get the issues you’re describing unless the receiver has misconfigured or over-protected their inboxes. And surely you’re not thick enough to think the same line of code on Proton’s side is responsible for sending an upgrade link in the web-app, and in an email. That means it’s a separate issue, a different bug, if you could even call it that.

Its seems like you’re just angry and feel like yelling at anyone else that doesn’t coddle you and your particular perspective. They’re trying to share their knowledge with you and you’re getting angry at them for that. Aim your anger at those who actually deserve it.