Email Subject Lines Best Practices

Email Subject Lines: Best Practices for 2019

In collaboration with Phrasee.

Why do email subject lines matter?

Email subject lines are one of the most important elements of any marketing email. In many cases, an email's success or failure hinges in large part upon the quality of its subject line. There are a few reasons for this:
  1. Industry-wide open rates hover well below the 30% mark (and in most cases, far below it). The email subject line is the only part of many marketing emails that most subscribers will ever see. Those impressions count.
  2. As many as 47% of email users report that the email subject line is the most important factor in making the crucial decision of whether or not to open an email.
  3. Modern inboxes are crowded places because many brands are vying for the same subscriber attention. A well-crafted email subject line is often your best chance to stand out from the crowd and drive engagement.
  4. Do the math: Better subject line language + effective split testing = more opens, clicks, conversions and revenue.

Subject Line Best Practices in 2019

The ways in which consumers interact with marketing emails and their subject lines seem to change with the weather. Thus, making it difficult for many brands to keep up with the trends. However, there are always best practices that brands should be putting in place to ensure that they are getting the most out of their email subject lines. Here’s how email subject line best practices are shaping up in 2019...


People love to focus on email subject line length as an important factor in determining its performance. There are plenty of blog posts out there on the subject. Why? Because unlike the many other elements that make up a subject line, the length is the easiest one to measure! The truth, however, is that the actual impact of email subject line length on email marketing performance is far smaller than people realize. There are effective short email subject lines and effective long email subject lines, and everything in between. Don’t restrict yourself due to pre-conceived notions and flawed data. When it comes to building a good email subject line that generates results, there are much more important things to worry about than length.


Nobody likes being tricked. Using spammy, dishonest messaging in your email subject lines is bad for business. While including overly generous offers in email subject lines may generate a short-term uplift in opens, consumers catch on to such spurious tactics quickly. If the value you promise in your email subject lines doesn’t match the value you deliver in your email’s body copy, all you’ll succeed in doing is annoying your subscribers and making it less likely that they’ll open your emails in the future. Worse still, such tactics can land your emails in the spam folder or cause an increase in unsubscribes. Protecting your sender reputation and your long-term relationships with subscribers is worth a lot more than any short-term uplift in performance that dishonest subject line language could ever offer.

Split Testing

If you aren’t already running split tests on your email subject lines, you should be. There is some value in traditional “A/B” subject line testing to determine which one is most effective on an audience. However, more robust “multivariate” split testing models involving several subject line variants yield much more useful data. Here at Phrasee, we typically run split tests on 10 subject line variants for every campaign. Ongoing multivariate split testing is the most effective strategy for determining what kind of subject lines work most effectively on your brand’s unique audience. Just remember to make sure that you have a robust system in place to learn from the results of your split tests so that you can apply those learnings to your future subject lines. Email subject line split testing best practice is to always optimize towards increased email open rates, rather than further-down-the-funnel metrics like clicks or conversions. Statistically, optimizing towards clicks or conversions rather than open rates reduces long-term expected profit. So, stick to open rates as your subject line performance metric of choice.

Language Diversity

Re-using the same words, phrases and syntax, or targeting the same sentiments in your email subject lines will offer diminishing email marketing performance returns over time. Experimenting with new language elements and testing out new ways to deliver your brand’s messages through diverse email subject line language is a smart approach to email marketing. Phrasee’s data has shown that split testing email subject lines using a diverse set of subject line language will, on average, lead to an open rate uptick of 15% - 20%.

Avoid Spam

Fact: nobody likes spam. That’s why constructing your brand’s email subject lines to look as un-spammy as possible is crucial to email marketing success. To do this, you must write in your brand’s tone of voice and remember that there is a human being on the other end of this message. While “spamminess” as a quality of language is difficult to quantify, we all know it when we see it. Subject lines in all CAPS, excessive use of punctuation, and other lazy attention-grabbing tricks are all generally bad ideas. However, every audience is different, and it is only through experimenting and ongoing split testing that you’ll learn where your audience’s “spammy” boundaries lie.

Artificial Intelligence

The ongoing writing, split testing, and optimization of email subject lines are time-consuming and difficult tasks. Luckily, artificial intelligence has made doing so more practical and effective than ever before. Phrasee is the world’s most effective AI-powered marketing language generation and optimization tool. It was designed to apply data science and evidence-based reasoning to the problem of writing email subject lines that drive consumer engagement and generate the opens, clicks, and conversions that make modern email marketing successful. Some of the world’s biggest brands have already implemented AI-driven solutions into their email subject line strategies, and the results have been impressive, to say the least.

A Chat with Phrasee

Parry Malm is the CEO and Co-Founder of Phrasee. Together with Chief Scientist, Dr. Neil Yager, and COO Victoria Peppiatt, he founded Phrasee in 2015 to develop a practical AI-powered application that could write higher-performing email subject lines than human copywriters. Since its founding, Phrasee has had a hand in more than 10,000 email marketing campaigns across dozens of industries. To date, Phrasee-generated and optimized subject lines have appeared in the inboxes of 44 million individuals across the globe. We recently sat down with Parry Malm and Dr. Neil Yager to chat about email subject lines, and how artificial intelligence is helping them deliver better marketing performance for brands.

1. Do brands put enough thought/effort into creating email subject lines?

Parry: Yes and no. People put a lot of thought into their subject lines – but more often than not, it’s not thinking and time well spent. People rarely follow a structured process to come up with new subject lines, and when they do, their internal heuristics are over-run with pre-existing bias. There are machines that are more suited to the task than humans. The humans who realize this will free up their time and thought processes for other tasks, and will let the machines take care of the subject line.

2. What kind of impact do email subject lines have on unsubscribe rates?

Neil: The biggest risk is using misleading subject lines. As an example, consider a subject line that says “Neil: regarding your recent order” and the email body contains “We’d love for you to place another one!” The open rate for this email would probably be very high since people are likely to think that the company is trying to get in touch with them specifically. However, a lot of recipients would unsubscribe after they realize it was just a cheap trick. In order to keep unsubscribes to a minimum, make sure that the body of an email follows naturally from the subject line.

3. Couldn’t using artificial intelligence technology on something as simple as an email subject line be considered overkill?

Parry: Couldn’t NOT using a technology proven to get >20% more eyes on your offers be considered irresponsible? Brands that are making use of AI-optimized email subject lines are seeing massive benefits. Virgin Holidays, for example, credits Phrasee-generated subject line copy with revenue growth in the millions of dollars.

4. Why split test email subject lines to optimize open rates? Wouldn’t it make more sense to optimize for further-down-the-funnel metrics like clicks and conversions?

Neil: The ultimate goal is to increase revenue. There is no doubt about that. However, there is a practical reason why open rates are so important. Conversions are relatively rare events, making them more susceptible to being swayed by randomness and noise. Perhaps there is someone on the list who places a single large order every year. They are randomly assigned a test group, making that whole segment look artificially strong. Open rates are a more robust measurement since the sample size (the number of people involved) is orders of magnitude higher than the number of people who click or place an order. However, open rates are a good proxy for clicks and orders, since opens are a necessary step in the sales funnel. To make a long story short, under most circumstances, the split test open rate is the strongest indicator of an email campaign’s revenue potential.

5. Should brands be using emoji in their email subject lines in 2019? Any specific tips on how to do this effectively?

Parry: Emojis are but one form of language used in a subject line. You could also ask: should brands be using imperative verb tenses? Should they be using second person personalization? What about discounts? Or multi-clausal phrase structures? The answer to all of these questions is yes… and no. No one linguistic feature is the secret sauce. Much like anything else, emojis exist within the context of a larger piece of language, so isolating it as a simple “emoji or eNOji” question is a fool’s errand. 

6. What kind of impact do email subject lines actually have on a brand’s bottom line?

Neil: A lot of effort goes into creating an email marketing campaign, from the design of the body copy to the landing page and offers. However, all of this effort is for naught if an email remains in an inbox unopened. Through tracking billions of human interactions with email subject lines, we’ve found that using high-quality, diverse subject line language can boost the revenues generated from an email campaign by more than 20%.

Keep Following the Chat with Phrasee

We’ve only just scratched the surface with subject line best practices and motivating opens. Catch us on Twitter Tuesday, July 9th when Phrasee and Email on Acid sit down for a Twitter chat about all things inbox displays, from subject lines to sender “from” names.
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