From experience (in the past I managed a mailing list with over one million subscribers), the only good list of emails is one where everyone has done a double-o

From experience (in the past I managed a mailing list with over one million subscribers), the only good list of emails is one where everyone has done a double-opt-in, a.k.a. confirmed opt-in. We had to do it this way, not only because it is law in many countries (and the site offered legal advice!), but because when running a mailing list that large, you need to do everything you can to run it properly: because you will inevitably run into problems with various ISPs spam traps, having double-opt-in makes it a little easier to get someone to un-ban your servers if/when the situation arises. But I digress. I agree with a previous commenter, Piskvorr - send everyone a mail asking them to confirm their subscription. FWIW, we didn't use captchas - we didn't see the need as we were using double-opt-in (analytics said only a tiny percentage of sign-ups on our site didn't confirm) edit: I meant to include this link https://en.wikipedia.org/wiki/Opt-in_email#Confirmed_opt-in_.28COI.29_.2F_Double_opt-in_.28DOI.29 https://en.wikipedia.org/wiki/Opt-in_email#Confirmed_opt-in_...