Data-Backed Email Subject Line Optimization Tips & Best Practices

by Michael Bibla in Conversion Optimization

Staying on top of your emails can be difficult. There is not enough time in the day to engage with all of them.

For digital marketers, this means you must choose your words wisely to craft email subject lines that will get more people to open the message.

The importance of email subject lines

Your subject line may only be a few words, but it’s the only chance you have to convince recipients that your message is worth reading. After all, 93% of B2B marketers credit email as the most effective distribution channel for demand gen efforts:

email subject line optimization distribution

It doesn’t matter how good the content is if no one ever sees it. So, carefully writing, testing, and optimizing subject lines has a sizable impact on your open rates.

What is email subject line optimization?

Subject line optimization is loosely defined as the process of testing, analyzing and updating email subject lines. The goal here is to identify which elements of your subject lines your audience responds to the most.

Here is a subject line that needs optimizing for a few reasons:

  • Too long — two full sentences is overwhelming
  • Not personalized in any way
  • Too many potential actions (call or discuss the report)

email subject line optimization improvements

Every marketer’s audience is different and what appeals to them will change. However, some general trends can result in better subject line performance. From best practices to A/B testing to data analysis, there are many ways you can optimize this critical point.

Using our data analysis at Atomic Reach, we suggest the following best practices for high-converting subject lines.

Word count

What drives more opens, shorter or longer subject lines?

Subject lines with exactly 5 words generate a 15.9% increase in open rates over longer ones. Here are a few examples:

  • “Fly South for the Summer.”
  • “Swipe right on these deals.”
  • “Love your career? Stay certified.”

Higher open rates could be caused by a few trends. First, the amount of content consumed on mobile devices has increased significantly over the last few years. According to Campaign Monitor, mobile users check their email three times more than non-mobile users. Moreover, a few impactful words are easier to read than a complete sentence, and recipients are more likely to engage with the email as a result.


American Express hired psychologist Emma Kenny to conduct research into how they could best engage with millennials. They discovered that millennials were more likely to engage with a personalized offer compared to other age groups.

Now think about your inbox, and odds are it’s flooded. To stick out, get noticed, and opened, you could include a personalization token to make the experience with your email unique. In fact, subject lines with a personalization token can generate 23.6% increase in open rates on average compared to those without a token:

email subject line optimization personalization token


Many brands include pronouns in their subject lines to connect with the recipient. The thought process is, by addressing them with a pronoun, it makes the email seem conversational and human.

For example, “Let us travel with you!” or this:

email subject line optimization pronouns

However, once interpreted as a human approach to email marketing may no longer be the case. It turns out, data at Atomic Reach shows that subject lines with no pronouns generate a 52% increase in open rates on average rather than subject lines with pronouns.

This is another data driven example of how our human nature may influence how we craft subject lines. The same goes for including numbers.


Numbers and data don’t always tell the whole story. Subject lines that contain a number only generate on average a 3% increase in open rates when compared to subject lines that omit numbers.

As prospects inboxes continue to overflow, they may start filtering out emails with numbers as they can be viewed as spam.

If you’re going to include numbers, be sure to use compelling data points that enhance the email’s context. While these are best practices to go by, you might find that your industry or audience responds better to variations of these rules.

The best way to find out what appeals most to your audience is to conduct a full audit of your email campaigns. This will give you a deeper understanding of which elements of your emails are driving engagement.

How can AI impact your subject lines?

Data analysis

This may come as a surprise to many, but for differing subject lines, there are platforms and machines that can write better subject lines based on data.

As artificial intelligence technologies continue to evolve, they can analyze data at a rate human never could. These platforms continue to learn what performs well over time and can implement it into applications quickly.

For a human to analyze 10,000 subject lines, for example, they would be staring at spreadsheets for days trying to determine what common factors are contributing to email open rates.

Compare that to an AI-powered platform that can analyze thousands of subject lines in minutes, and use the insights to craft compelling subject lines that will outperform previous tests.

Lack of bias

Humans also have an inherent bias, we get protective of our creative ideas and even if the data is there to tell us we’re wrong, sometimes it’s difficult to recognize.

An AI-powered platform can analyze without bias. In most cases, it is better equipped than humans to test, analyze and extract insights from large datasets.

Keep your brand tone

Tone is a particularly difficult element to measure.

Leveraging artificial intelligence allows you to analyze brand tone throughout all of your previous email subject line data, and then implement it consistently moving forward. It can use this information to determine your subject line’s tone so that when creating new ones, each greeting aligns with your brand.

Will AI replace humans?

Of course, there are AI technologies that can drastically increase your open rates through their data analysis. But, for them to create high-converting subject lines you have to train them first.

Writing email subject lines is still a unique process that requires the human touch, but AI can assist in the process, saving time and money while providing you with further insights. With the endless stream of inbound content sent to inboxes every single day, having a compelling and click-worthy subject line could not be more valuable.

By using best practices, analyzing data and embracing new technologies like AI, you give your emails the best chance of being opened and connecting with customers.

In addition to writing effective email subject lines also remember to connect your email offers with personalized post-click pages. Sign up for an Instapage Enterprise demo here.

About the author
Michael Bibla is the Content Strategist at Atomic Reach, a content intelligence platform used by marketers to generate more leads from their text-based content. He is on a mission to empower marketers with actionable data so they can make more informed decisions about their content strategy.

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