Another issue with Sign in with Apple is the fact that their private relay has a pre-set allow-list per app for sending email to relay addresses. This means th

Another issue with Sign in with Apple is the fact that their private relay has a pre-set allow-list per app for sending email to relay addresses. This means that you must either prove ownership of domains, or pre-add email addresses to Apple's systems. I understand why they have done this, it will reduce spam considerably, but the private relay system is already designed to empower users to do this and this extra step may be impossible for some developers. Take for example a retailer – they need to dispatch goods and use different carriers in different countries. When the user buys something they very likely want email notifications about delivery, a feature that most carriers provide. For the carriers to send those notification emails you'll need to pre-add them all to Apple's systems. You can't prove domain ownership because fedex.com isn't your domain, but where are those emails going to come from? Better hope your carrier doesn't change sending address at some point or the email goes into a black hole. Apple also limits the number of domains and addresses you can send from. In the original documentation it was "10 domains and addresses" (not sure if 10 of each, or 10 total). This was raised to 100 I believe, but that's still probably an issue for larger multi-national companies, or those who necessarily have to integrate with many external services. The really hard-line privacy stance is that the retailer shouldn't share the emails and should do the notifications themselves, but for many this is prohibitively difficult to do, or at least detracts from places where the retailer can actually add value. The benefits are also very small, as the contracts with carriers typically protect user data, require deletion quickly after delivery, and retain most privacy benefits while allowing for a good UX.