Apple not sharing data with third parties

Actually I found my answer right on their website. Apple’s “Hide My Email” and/or “iCloud Private Relay” seem to be the culprits!

RentalCover.com

How to Activate RentalCover with Apple ID or Hide My Email

Can't find your RentalCover confirmation email? Learn how to activate your account and fix login issues caused by Apple’s Hide My Email...