How to write the best email subject lines and boost engagement

Email subject lines are the first thing recipients see when they receive your email, and this messaging plays a major role in determining whether or not the email gets opened.

A great email subject line, along with the preheader text, should grab the reader’s attention, provide a hint of what’s inside and motivate the reader to take action. So, if you want your emails to be read, optimizing your email subject lines is essential.

We’ve identified three guiding principles to help you write your best email subject lines: 

• Personalize it

• Make it relevant

• Test to optimize

In this guide, we’ll show you how to optimize your email subject lines, and provide professional email subject line examples that will help you boost your open rate. Me-ow!

We searched millions of emails to find effective email subject lines. These lines were chosen because they had exceptional open rates. While the content is specific to someone else’s newsletters, you can see what works for them and apply it to your own topics.

Our data scientist analyzed millions of email marketing campaigns to unearth new insights about effective subject lines. Here’s what he found:

Subject line keywords that convert

These are keywords used in subject lines that tend to repeat in well-performing campaigns.

High-performing keywords: “welcome”, “ebooks”, “bargain”, “video”, “bonus”, “party”, “daily”, “updates”, “event”, “challenge”, “important”, “surprise”, “meeting”, “announcement”, “final”, “favorite”, “exclusive”, “discount”, “stream”, “confirmation”, “requirements”, “premiere”, “briefing”, “confirm”, “promotion”, “downloading”, “recording”, “congratulations”.

Takeaway: Welcome/confirmation emails perform exceptionally well, make them count!

This chart shows open rates when using keywords associated with dates.

Poorly-performing subject line keywords

These keywords that raise a sense of urgency tend to repeat in very poorly-performing email campaigns.

Subject line keywords that don't perform as well: “missed”, “grab”, “easy”, “case”, “email”, “resending”, “post”, “game”, “forget”, “flash”, “rate”, “enter”, “loan”, “resend”, “resources”, “activities”, “article”, “unlimited”, “career”, “level”, “influencer”, “wicked”, “jump”

Takeaway: If you’re resending a campaign, don’t tell subscribers in the subject line that you’re resending them recycled content. Get creative and encourage them to open with a subject line that’s more likely to convert

Subject lines with numbers

We’ve also found that open rates are affected when numbers are used in the email subject line.

Takeaway: Numbers can increase open rates if used properly and for certain industries. See below.

• Numbers help these industries: Open rate significantly increased (by 2% or more) for Online courses, Automotive, Software and web apps, Information technology (IT), Politics, Art and Artists when numbers were used in the subject line

• Numbers don't help these industries: Open rates are decreased for Galleries, Museums, Mobile industries, Music and Musicians when numbers were used in the subject line

1. Tell readers what your email is about

It might sound like obvious advice, but it bears mentioning. You’d be surprised how many people forget to even elude to what their email newsletter contains.

You want to tease the content in your subject line to pique interest and encourage subscribers to read more. If you’re too vague, there’s no incentive to open and take action.

In the first example above, it’s clear what the reader will gain from opening the email. The second example makes an attempt to generate FOMO but neglects to include any hint as to what the benefit of reading is.

2. Insert your reader’s name

When a reader sees their name in a personalized email subject line, something happens in their brain where they need to see what it is. This level of personalization is personalization is easy to implement easy to implement and available to every MailerLite user.

3. Use AI for inspiration

Coming up with clever subject lines isn't easy. If you're stuck for inspiration about what to include, use AI to write crafty copy for you. You can use the generated content as is, or edit it to create a subject line that's sure to wow your subscribers.

Using AI to create subject lines is more convenient than ever thanks to our new AI subject line generator, which you can find in the email details section of a campaign or automation. Just describe what the email is about, choose a tone and email type, then hit generate. The tool will suggest 5 different options for you to use.

4. Personalize the sender address

The first two things people look at in their inbox are the subject line and the sender's name. Use a familiar name. If possible, send the email from a real person. 

You don’t care what is the subject line of the email you received from your mom, right? That’s the point. We usually open and read emails that are sent by someone we trust and know. 

For example, 'Jonas from MailerLite' works well because the reader will recognize the company and the name adds a personal touch.

When an email is sent from a familiar sender, opening rates increase by as much as 28%. In a world of endless spam and email bots, personalizing your sender address breaks through the inbox clutter. 

5. Refer to your reader's location. 

People feel a connection to their hometown. When you include location-specific language in the subject line, your readers will be curious about what is happening in their area.

6. Highlight your reader's interests. 

If you have collected data on your users, find ways to connect with them through their interests. For example, you can segment your audience by their favorite sports team. 

By calling out something meaningful to your email recipients in the subject line, they are more likely to open the email.

7. Make it relevant

Email marketing that is relevant means sending the right messages to the right people. Tailor your content to your reader’s interests and desires.

Instead of blasting out an email to your entire email list, you can increase relevance by segmenting your audience into smaller groups. When you send emails to smaller portions of your list, you can get more specific based on that group's interests or needs.

Not only are readers turned off, but it also indicates that your content is not relevant enough to explain it simply.

Which headline is more descriptive?

Subject line #1 is not very descriptive and makes a big claim with nothing to support it. On the other hand, line #2 lets the reader know exactly what to expect in the email.

The best subject lines make a promise and then deliver on that promise. Of course, your content must deliver on the promise, or your bounce rate will go through the roof.

8. Avoid looking like spam

Don't use spammy phrases that sound too salesy or make promises you can't keep. Readers are savvy to clickbait and will immediately mark your email as spam. Do your best to minimize exclamation points or CAPS. People are usually not as excited as you are about your email. In fact, our data found that subject lines that insist on urgency perform much worse. 

9. Consider responsive mobile display

Over half of your subscribers are viewing emails on mobile devices. The subject line parameters change with mobile. iPhone cuts off subject lines after 35 characters but gives 140 characters of preheader. Keep your subject lines under 40 characters to ensure it's readable on every device.

10. Ask a question

People like answering questions. Questions draw the reader into the conversation as a participant. Try subject lines that pose a question to pique curiosity. Your email open rates are likely to increase as people naturally want to find answers.

11. Use numbers and lists

For some reason, people love to click on emails with numbers and lists. Perhaps it's because their expectations are clearly defined or it's easier for the brain to process. Whatever the reason, numbers and lists have been proven to work. Our 2020 data shows email subject lines using numbers generate a 24.97% open rate versus 23.34% without using numbers.

12. Leverage scarcity

Have you ever noticed how Apple sells out of their newest products forcing most to wait? Apple has mastered the scarcity principle, where the value rises when demand is greater than supply. You can use this same strategy in your subject line. Let readers know that your offer is limited or has an expiration date.

13. Experiment with emojis

In some industries, emojis are a total hit. Research and testing have shown that emojis can increase open rates and click-through rates, make your email stand out, and give your brand some personality.

But emojis don’t always translate. Some email marketers find that emojis can drastically decrease email metrics. To find out if emojis work for your audience, A/B test your subject lines with and without emojis.

We actually looked at 48 industries and compared the open rates of newsletters that included emojis in subject lines versus those that did not. Check out the results in our complete guide to using emojis in subject lines.

14. Use preheaders to your advantage

The preheader is your subject line's best friend. It's the first line of text that appears next to the subject line. This is especially important for mobile devices where the preheader is often more prominent than the subject line.

For example, iPhone only allows 35 characters for a subject line, but displays about 140 characters for the preheader. That is a full tweet! (Well, an old tweet before Twitter increased it to 280 characters).

Seize the opportunity to include compelling text that communicates your value proposition and motivates your readers to open the email.

15. Give a command

Sometimes your readers need to be told what to do. Giving them clear direction and a strong call-to-action will actually help them take action. The trick is to give a directive without being too bossy. 

16. Test, test and test again

Even if you were to implement all these tips to the letter, there’s no way of knowing that they will all resonate with your audience and improve your email marketing strategy without testing.

Email A/B testing allows you to split your audience and test two different subject lines. This lets you gain valuable insight into what works and what doesn’t, so you can create more effective subject lines in the future.

The best part is, A/B testing doesn’t begin and end with subject lines. You can A/B test preheaders, email body content, sender address and more! Just make sure you only test one element at a time. That way you get a clear idea of which tactics improve performance.

Each email subject line you create has one sentence, one chance, to make the right impression and get subscribers interested. Much like our new friend Fluffy, if they enjoy the first meeting they’re more likely to be open to the next one. From there you can start building a strong long-term relationship with your subscribers (and no one is at risk of getting scratched).

The email subject line and preheader should work together to build anticipation of your valuable content. To learn more, check out this article where we cover everything you need to know about email preheaders.