Apple has deployed a system called Private Access Tokens that allows web servers to verify if a device is legitimate before granting access. This works by having the browser request a signed token from Apple proving the device is approved. While this currently has limited impact due to Safari’s market share, there are concerns that attestation systems restrict competition, user control, and innovation by only approving certain devices and software. Attestation could lead to approved providers tightening rules over time, blocking modified operating systems and browsers. While proponents argue for holdbacks to limit blocking, business pressures may make that infeasible and Google’s existing attestation does not do holdbacks. Fundamentally, attestation is seen as anti-competitive by potentially blocking competition between browsers and operating systems on the web.
These days you can spin up a docker container with all the email trust stuff preconfigured
And the emails from your custom server will be rejected and/or marked as spam by every major corporation, and you will spend countless days writing appeals and appeal rejects