One would imagine that the majority of us have received a bit of spam here and there, and probably don't want to help you out here. It isn't terribly hard to write a quick mass mailing script - I would expect anyone with about 20 hours of database scripting work behind them to be able to do so - but the purpose is probably not going to make you any friends and, like you said, it's not really related to pentesting. That said, I've never actually tried it, but I would probably be taken aside for a stern ass-kicking if I tried this during a pentest. A few users sure, but over 100 would be a bit rough.
There are a lot more programs out there than just ENS - try searching for them. They're called stupid things like "Client Newsletter Mailer" and "bulk email distributor".