Your email address is an excellent identifier for tracking you across devices, websites and apps. Even if you clear cookies, use private browsing mode or change devices, your email address will remain the same.
Due to privacy concerns, tracking companies including ad networks, marketers, and data brokers use the hash of your email address instead, purporting that hashed emails are “non-personally identifying”, “completely private” and “anonymous”. But this is a misleading argument, as hashed email addresses can be reversed to recover original email addresses. In this post we’ll explain why, and explore companies which reverse hashed email addresses as a service.
Source: Four cents to deanonymize: Companies reverse hashed email addresses