if you have your own mailing list, emails work very well (it's double-opt in). My ex-boss bought $5000 worth of mailing lists against my advice. The result wa

if you have your own mailing list, emails work very well (it's double-opt in). My ex-boss bought $5000 worth of mailing lists against my advice. The result was a .0001% click through rate and a banning from our server provider. Unless the lists are tailored to your industry and you know exactly how they were obtained (which is nearly impossible), it's not worth it. You can try to advise your client. But like my boss, they may just need to make the mistake themselves.