Opensource

That's what you are paying for. You can't really scrape email addresses. So that database is marketing data that is bought and sold.

I own my own domain. I have email hooked up to that domain. I'm the only user on my domain. ALL email addresses get forwarded to me. FenderStratocaster@mydomain.com would show up in my email box.

Corporations also get every email address for their domain. They don't broadcast valid email addresses. Typically they receive a valid one, and forward it to the proper person inside their org. Or if invalid, they will discard it. Some places will return a response saying that's not a valid email, but most people stopped doing that. One because it costs money, and two it's actually a security hole. You could theoretically brute-force valid email addresses of top executives, or anyone.

Instead websites gather lists of addresses by means of new accounts, and sell that data to these companies, who sell it to people who want it.

You need the database to get the into.