MD5 of a normalized email, `MD5(NORMALIZE_EMAIL(email))`: lowercase hex, 32 characters. Hashes of un-normalized emails won't join. The MD5 of an empty string, `d41d8cd98f00b204e9800998ecf8427e`, isn't a hash of any email and reads as NULL.
An email address hashed with one of several message-digest algorithms; the specific algorithm is carried in the type property of each value. Prefer the algorithm-specific attributes when the algorithm is known: md5_hashed_email (74), sha1_hashed_email (75), sha256_hashed_email (76). Hashed emails only join when both the algorithm and the input normalization match.