Over the last year, I have taken my 'client' (technically, I'm an employee) from black hat email marketing practices (mining for email addresses by industry category and sending them out from our own server) to building an opt-in subscribe list and using a great ESP like Campaign Monitor to send the campaigns.

After a recent failed auction event (that's what we do; auctions), there is going to be higher-ups asking **why not go back to the old way of sending out to tens of thousands of mined email addresses. **

I could use some ammunition of why that is a bad idea. Any resources or advice would be greatly appreciated. Thanks!