Some quick thoughts on this...
Do not make stupid decisions. Use common sense. Know the boundaries. Be safe. You are responsible for *your* actions.
1) Familiarize yourself with black hat data dumps, specifically in the dark web. The idea here is for you to search as many stolen data dumps as you can, looking for your target domain(s). For example:
1.a) If you are targeting domain.tld and want to find actual email addresses used with this domain, for example
[email protected] and
[email protected] then be on the lookout for data dumps.
1.b) If a data dump takes place, perhaps from a major streaming service, then you would locate that data dump and search it to see if there are any hits for "@domain.tld".
2) https://www.kali.org/tools/emailharvester/ :: https://github.com/laramies/theHarvester :: https://github.com/nettitude/Prowl :: https://github.com/righteousgambit/quiet-riot :: https://www.geeksforgeeks.org/smtp-enumeration/
BTW, you may want to learn more about "email enumeration" or just "enumeration" in general.