Email Marketing
Email subject lines is always of interest as marketers try to get through inbox clutter. A couple of recently-released studies provide some fresh data to review.

(The following results will not apply to all readers, who are encouraged to test their subject lines.)

A study from Return Path, based on an analysis of “9 million subject lines received by more than 2 million subscribers from January 1st through February 28th, 2015 using its Subject Line Optimizer solution.”


The analysis finds no correlation between subject line length and read rate. That stands in apparent contrast to research from MailerMailer last year, which had linked shorter subject lines to higher open and click rates (though significance was not tested).

In the Return Path study, subject lines with 61-70 characters had the highest read rate, of 17%, almost twice the read rate of subject lines with more than 100 characters. Both were not heavily represented, though, comprising just 6% and 3% of messages analyzed, respectively. The most commonly used length was 41-50 characters, in one-quarter of emails analyzed.

The researchers caution that just because there isn’t a relationship between subject line length and read rate doesn’t mean that marketers should not pay attention to length. That’s because different devices will display subject lines in different ways, with desktops typically showing about 60 characters and mobile devices 25-30. As such, it’s more important to place a call-to-action at the beginning of a subject line if the audience is  mobile.

Here are some of the terms associated with the highest and lowest read rates:

  • Benefit terms, subject lines containing the term “fastest” had the highest read rates (+5.3% above-average) and “cheapest” the lowest (2.94% below-average);
  • Clickbait subject lines generally don’t do well, surprisingly, with “secret of” terms (-8.69%) having a significantly below-average read rate;
  • Command keywords (“telling subscribers what to do”), subject lines with the term “register” (+6.7%) had easily the highest read rates, while those with “put” (-1.5%) and “buy” (-1.25%) had below-average rates;
  • How-to terms in the subject lines tended to perform right about average;
  • Discount-related emails generally had below-average read rates, particularly the case for those with “2for1″ in the subject line (-6.62%);
  • News-related subject lines didn’t perform too well, and those containing the term “announcing” had read rates 1.45% below the average;
  • Pricing-related emails, subject lines containing “%,” “$,” and “free” all had below-average read rates, though the study authors surmise that the size of the discount or value of what’s being offered for free likely have a bigger impact on the read rate;
  • Reason why terms had mixed results, as those containing the term “steps” performed above the average (+1.23%) while those containing “here’s how” fell 1% below-average;
  • Urgency terms, with those including “still time” performing way above-average (+15.54%), even as those with “running out” in the subject line fared poorly (-3.3%); and
  • Pronouns such as “he,” “I,” “mine,” and “it” fared at or slightly below-average (with “mine” the furthest below-average, at -1.69%); the term “you” was the only pronoun to be associated with an above-average read rate, albeit a very small lift (+0.1%).

In a separate study, Phrasee takes on email subject line analysis via a look at 700 million emails in the retail and e-commerce sector, primarily from the US and the UK. The analysis generated a “Phrasee Score” for each term, with higher scores indicating more reliably positive results from a term or phrase.


  • Experiential terms such as “celebrate” and “love” tend to have a more positive effect than “spend” or “grab;”
  • The term “delivery” generally works better than “shipping,” and the phrase “plus free delivery” delivers a high Phrasee Score;
  • “New products” and “new lines added” outperformed “new arrivals” in the analysis;
  • It’s “hit or miss” when it comes to percentage discounts;
  • Including the brand name in the subject line may be a good thing; and
  • Email subject lines containing questions that can be answered with a yes or no tend to do better than open-ended questions.

To add to the personalization body of research, the phrase “just for you” generates a higher Phrasee Score than the inclusion of the first name in the subject line.

About the Data: Phrasee methodology: This analysis is derived from anonymised response metrics that email marketers have inputted into Phrasee. It’s based on roughly 700 million emails, all in the retail and ecommerce sectors, primarily from the USA and the UK, with a few Canucks, Ozzies and Kiwis thrown in for good measure. It is all very recent data from the last 2 months. We then took the individual phrases and ran millions of Markov Chain Monte Carlo simulations. This allowed us to create millions more potential outcomes that expanded the data set much further beyond the original 700 million. We time-weighted the training data and built random variance into the model to control for experimental bias. This method is much stronger than purely looking at past results: it allows us to create a predictive model for subject line success… and thus the Phrasee Score.

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