I think the concern may be that the total space of hashes for valid email addresses, especially ranked by domain usage, is small enough that SHA hashing doesn't

I think the concern may be that the total space of hashes for valid email addresses, especially ranked by domain usage, is small enough that SHA hashing doesn't meaningfully obscure content. From the article: Hash It! The first instinct is often just to hash the contact information before sending it to the server. If the server has the SHA256 hash of every registered user, it can just check to see if those match any of the SHA256 hashes of contacts transmitted by a client. Unfortunately, this doesn’t work because the “preimage space” (the set of all possible hash inputs) is small enough to easily calculate a map of all possible hash inputs to hash outputs. There are only roughly 10^10 phone numbers, and while the set of all possible email addresses is less finite, it’s still not terribly great. Inverting these hashes is basically a straightforward dictionary attack. It’s not possible to “salt” the hashes, either (they always have to match), which makes building rainbow tables possible.