Earlier this week,  I wrote about three great tips for mobile-optimized emails, an important topic for any subscription content site since email is the most cost-effective way to acquire new subscribers, according to our 2013 Online Subscription Benchmark Report.But email is almost more important when it comes to retention marketing (with one great on-site exception). That’s why we thought this renewal email from Carbonite was so effective — it had three advanced tactics that subscription sites should consider adopting:#1. Remind your members what they have with you. Instead of just highlighting “great content,” renewal emails should make it seem like NOT renewing is a bigger headache than renewing. Carbonite does it below by reminding a subscriber that he has “123,918 files” that he may want to keep. That’s a huge incentive. Subscription sites could do the same by allowing subscribers to create personal and annotated content libraries, member profiles (that link to others), badges and status recognition within a community.#2. Hyperlink to a renewal page in the pre-header text. As I mentioned on Tuesday, smart use of the pre-header text is great for mobile opens, engagement and even conversion (although conversions on a mobile device are always lower than desktop averages). Carbonite mentions the file size first in the pre-header text, then has an underlined hyperlink saying “RENEW EARLY.” Then, instead of the long-winded “Problem viewing this email? Click on the mobile version,” Carbonite concisely says “view as webpage.”#3. Use a clear CTA with supporting text and sense of urgency. Research has shown that have supporting text below a call-to-action button that conveys urgency can lift conversions. Carbonite does this with button copy that says “Renew Now” and supporting text asking subscribers to “Act Now!” and then having an expiration date for the offer. Two in one!Click to enlarge the image below.