@varx yeah, it's a tricky problem. I'm currently looking into using k-anonymity to probabilistically anonymize the list. Ie, each recipient will have to do O(log(n)) checks to confirm their number, but an attacker will never be able to get an exact match.
Or set up some kind of shared secret or other anonymized id to use instead of the real id. This is actually a much simpler approach, but due do where the API layers land wrt ecapsulation, it's harder to design the software so each API layer makes sense.
Ugh, why is everything so difficult? :blobcat_thisisfine: