The Naz.API dataset is a massive collection of 1 billion credentials compiled using credential stuffing lists and data stolen by information-stealing malware.
Credential stuffing lists are collections of login name and password pairs stolen from previous data breaches that are used to breach accounts on other sites.
Information-stealing malware attempts to steal a wide variety of data from an infected computer, including credentials saved in browsers, VPN clients, and FTP clients. This type of malware also attempts to steal SSH keys, credit cards, cookies, browsing history, and cryptocurrency wallets.
While it’s not formalised in the email specs, support for it is pretty consistent, and only needs to work for whichever provider you use