Also, keep in mind that this is a partial leak. The data was scraped from some leaky endpoint which was patched out before every user could be scraped. Only users who were in the partial leak received emails (I have two accounts, only one received an email). If you're a Substack user but didn't receive an email, I'd assume you're not in the leak. Troy Hunt should load it into HIBP eventually, and those concerned can check there if they don't want to seek the leak out on their own.
