Last week, The Atlantic’s Quartz site quietly posted on its tumblr page some insights into its marketing strategy.For those of you looking to save time, here are the highlights:1. Instead of requiring account creation and registration, the site moved to a simple opt-in form where a prospect just had to enter an email address. We definitely recommend this practice for free newsletters.2. While the site only required an email address, it still allowed for optional information to be entered. This is a smart tactic, that can be augmented by employing progressive forms, i.e., collecting information over your customer life cycle instead of trying to capture as much as possible up front.2. The site moved from double opt-in to single opt-in. This is a tricky tactic to evaluate. On the one hand, it probably really helped getting newsletters delivered.But on the other hand, double opt-in is known to decrease unsubscribe rates. So while Quartz doubled its opt-in rate in less than two months, it noticeably doesn’t mention what the unsubscribe rate was during that time. More importantly, what will the site’s unsubscribe rate look like over the coming months and years?Therefore, we recommend double opt-ins for free newsletters, especially since people can get put on so many email lists they are more likely to remember yours if they opt-in twice. But if you’re looking to grow a list ahead of an important event date or product roll-out, single opt-in might get you the numbers you need in a pinch. Otherwise I say wait before you copy Quartz’s strategy.As a paid content site, what do you prefer for email newsletters — double or single opt-in?