Well I've spent 8 Years with a company specializing in mass emailing, with 200 clients (various email list owners).
This thread is surreal.
The quality of data is key to monetization - we're not in 1994 or 1997 to spam random email accounts with little guardrails in place.
All the data need to be validated, fresh (ideally 3 - 6 months old max) and you need to have a proper setup in place.
Then there's a major difference in the setup, as of how to treat an actual opt-in list, and how to treat a cold email list.
Gmail is a special discipline in itself, especially after the G Update in December 2021.
Seeing this actually encourages me to set up my own emailing setup, which is in my back-log anyway.