Ory is the largest open source community in the world for cloud software application security. We maintain advanced open source security software solving authentication, authorization, access control, application network security, and delegation. Ory implements a variety of industry and best-practice standards including OAuth 2.0 / OAuth 2.1, OpenID Connect, Zero Trust Networking, Google Zanzibar Policy Framework, FIDO2 U2F, WebAuthn, TOTP, and more.

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You can give them instructions to find the emails through their apple account so you can look them up, or if your system uses the email user [XXXXXXX]@privaterelay.appleid.com in some way and displays it to the user you can ask them to pass that to you as well.