I assume you have a fakedomain.com, with a * that forwards to your own main email. If so I'd use a deterministic algo. Examples: 1) (kind of) irreversible: md

I assume you have a fakedomain.com, with a * that forwards to your own main email. If so I'd use a deterministic algo. Examples: 1) (kind of) irreversible: md5(service-domain) 2) easily reversible (for you): aes(service-domain) with a known key Make sure you don't have pii in your fakedomain.com.