For those who aren't aware, "single opt-in" means, in effect, one isn't required to "confirm" a mailing list subscription. For example: a "pop-up form" (what t

For those who aren't aware, "single opt-in" means, in effect, one isn't required to "confirm" a mailing list subscription. For example: a "pop-up form" (what this article is actually about) appears, you type in your e-mail address, click the submit button, and you are now added to the list. You might ask, "What is to stop someone from adding arbitrary e-mail addresses (such as those belonging to other people) to the list?". The answer is, of course, absolutely nothing. As you might then guess, this was abused so much that "double opt-in" emerged. Double opt-in requires a "confirmation". If you've ever subscribed to a public mailing list (such as LKML, for example), you've seen this in action. After sending an initial "subscribe" message (or subscribing via a web interface), you receive an e-mail message requiring that you "confirm" your subscription by replying to it. The purpose of this is to prevent exactly the type of abuse described above. MailChimp is, in my opinion, already one of the "worst" e-mail service providers, as far as the amount of spam goes. Just a week or so ago, I had an e-mail conversation with them about double opt-in and threatened to blacklist their entire IP ranges if it doesn't improve (NB: as part of $job, I manage several e-mail servers) -- this was after one of my own e-mail addresses was added to a new list without any opt-in whatsoever! Unfortunately, it looks like the level of spam will only be increasing and I suspect I will eventually have to do exactly that. If you're a MailChimp customer and you care about "deliverability" at all, I would recommend you to begin looking at alternatives. I would have made that recommendation anyways (before learning about this change) but now I really recommend it.