[throwaway198389]

As Spotify points out, and I've seen first hand, Apple prohibits you from allowing users to create accounts or enter contact information if your service has a paid option and you don't use apple's IAP.

So you couldn't even get to the point where you can send an email without first falling in line with IAP.

And according to my understanding of the Spotify timeline, and I haven't see this because I haven't published to the platform in a while, Apple now prohibits you from targeting your iOS users with upgrade communications (even if they didn't sign up via iOS, which they didn't since that's prohibited).