Wouldn't a hash work great for this purpose? I.e. 1. User requests for email alice@example.com to be removed from database 2. Company removes "alice@example.c

Wouldn't a hash work great for this purpose? I.e. 1. User requests for email alice@example.com to be removed from database 2. Company removes "alice@example.com" from 'emails' table 3. Company adds 00b7d3...eff98f to 'do_not_send' table Later on, the company buys emails from some other third-party, and Alice's email is on that list. The company can hash all the email addresses they received, and remove the emails with hashes that appear in their 'do_not_send' table. You'd have to normalize the emails (and salt the hashes), but seems doable?