In a rush to acquire customers through Twitter and Facebook, email marketing is often dismissed as outdated and out fashioned. Why craft an email when you can engage on social media?

But don't dismiss the ancient act of sending emails so quickly; Tweeting and "liking" may be flashier, but a recent McKinsey & Co. survey states that email is vastly more effective way to acquire customers. How much more effective? Nearly 40 times more than Facebook and Twitter combined.

All marketing emails, of course, aren't created equal. It's all in the subject line – whether or not a customer decides to open your email or trash it rests entirely on its clickability. Luckily, a new study uncovers the attributes needed to create an effective one. Retention Science analyzed 267 million emails sent across 543 campaigns over the past six months. If you want to reach customers through email, consider these tips when crafting your next subject line.

Related: The Generic Email Newsletter Needs to Die

Keep it between six to 10 words. Subject lines with six to 10 words perform best, generating a 21 percent open rate, well above industry standard. Those with subject lines containing five or fewer words ranked second with a 16 percent open rate, and those with 11–15 words returned a minimal 14 percent open rate. Despite this, the majority of emails sent (52 percent) had subject lines in the 11-15 word range.

Think about the device it will be read on. Thirty-five percent of emails are opened on mobile devices. Given that most smart phones only display five or six words of a subject line, being brief and concise is even more critical for mobile marketing.

Take a personalized approach. It's shown to help re-engage and retain customers. Of the email campaigns studied, those with the recipient’s first name in the subject line delivered a 2.6 percent increase in open rates compared to those without a name.

Related: Survey: Email Is 40 Times More Effective Than Facebook and Twitter

Consider referencing a movie or a song. A separate Retention Science study analyzed 3.7 million emails and 22 campaigns where movie names or song lyrics were referenced in the subject line and found they were opened 26 percent of the time, while emails with more traditional subject lines were opened 16 percent of the time. Again, this allows you to take a more personal approach, and target certain customer segments.

But reconsider flash sale campaigns. Nearly 80 percent of flash-sale email campaigns had subject lines in excess of 20 words; unsurprisingly, they consistently underperformed compared to campaigns with shorter subject lines. Their bad performance record may also be blamed on the frequency at which they're sent -- flash sale brands tend to email customers four to eight times a week compared to the industry standard of two to four times a week.

Related: The Most Unsubscribed Email Lists of 2013