Though an email newsletter doesn't sound like the sexiest marketing tool, it sure can be effective. According to a Direct Marketing Association study, every dollar spent on email marketing generates--on average--$38 (representing a 3800% ROI)! It's part of the reason we strongly encourage our content marketing clients to take a second look at their own newsletter strategy.

If you're planning to click send on an email marketing campaign, first take note of some lessons from brands that are doing it right (and the ones that aren't quite nailing it).

In a recent study, email marketing optimization firm Coherent Path analyzed 100 businesses in the Internet Retailer Top 100 list's emails to both customers and non-purchasers. Out of those 100 brands, Wayfair bested the competition overall, and Levi's scored perfect marks in purchaser experience.

So, what can you learn from these master emailers?

Personalization is Paramount

To make it work, you have to gather lots of data on your visitors--and use it wisely. Wayfair personalizes most of its emails (71 percent, to be precise) - so when you get their messaging in your inbox, it's customized to your needs and shopping activity.

More than 75 percent of the emails Levi's sent to purchasers were unique--meaning, they didn't get the same messaging as non-purchasers. This means reducing the amount of discount language by 75 percent compared to non-purchasers (because no one wants to hear about a sale the day after buying something).

Across all the brands studied, only 26 percent of all emails were unique to the purchaser, and retailers sent the exact same email on the sale day to purchasers and non-purchasers alike 60 percent of the time.

Find Your Ideal Frequency

As anyone who has gone on an email purging spree can attest to, there is certainly such a thing as emailing too frequently.

For Coherent Path, too frequently means more than once every day. Ballard Designs and Free People earned perfect marks for sending just under one email every day, whereas the most frequent emailer--Williams-Sonoma--sent 3.14 emails daily (seriously, lay off!).

There's also the case of emailing too infrequently--especially with purchasers. TJ Maxx, for instance, managed to both over and under email, sending more than three emails a day 26 percent of the time, yet reducing emails to purchasers by 80 percent. Purchasers are your most engaged customers: connect with them in between purchases with unique email content.

Do Sweat Your Subject Lines 

Last, but not least, just as headlines are important for articles, subject lines are important for emails. Specifically, matching the subject line to the content score high marks--just like with headlines, clickbait has worn out its welcome in subject lines. Wayfair earned good marks for matching the subject line to the email content more than half the time.

See the full study here.