How to write the best email subject lines to boost open rates

We'll share our guiding principles to writing effective subject lines and provide tips that will help you improve your open rates.

You've spent a lot of time crafting the perfect email. You're excited to share it with all your subscribers. There is one last step that can make or break your campaign—writing a catchy email subject line for higher open rates.

The subject line along with your preheader text compete with hundreds of other emails in your subscriber's inbox. If you get the messaging right, they will open your email.

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 give you our best practices and share some great email subject line examples.

Email marketing is one of the most personal forms of marketing communication. The ability to target individuals allows you to personalize your messaging which improves your conversions. 

Here are a few ways you can personalize your subject lines and make a stronger connection with your readers:

Insert your reader's name into the email subject line. 

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

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.

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 subscribers in the subject line, they are more likely to open the email.

Note: Personalization can generate great results, but you have to be careful NOT to use it every time you send an email. Too much will lessen the effect until the reader starts ignoring it.

Personalize sender address

You can have the most personal subject line, but if the email is sent from a strange email address, your email will end up in the trash, or even worse — marked as spam.

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. 

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. 

Sometimes personalization isn't the right approach for your campaign. If it’s not, your goal is to write effective email subject lines that are relevant to your target audience.

You need to clearly communicate something that the reader either really needs or wants. To be truly relevant, you need to be specific and descriptive in your copy.

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.

What is relevance in email marketing?

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.

You can also increase relevance by implementing automation triggers, which send people emails based on their behavior. For example, when a user reads your blog, you can automatically send them an email with more content related to the blog. You can't get more relevant that!

1. Consider 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.

2. 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 open rates are likely to increase as people naturally want to find answers.

3. 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.

4. Avoid looking like spam

Don't use phrases that sound too salesy or make promises you can't keep. Readers are savvy to click bait 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. 

5. 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.

6. Experiment with emojis

Know your audience and proceed with caution. Younger demographics love emojis. If you use them appropriately, they will help improve open rates. More professionally-minded audiences might NOT respond well to emojis. 

7. 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. 

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: “update”, “bulletin”, “issue”, “breaking”, “historical”, “engaging”, “spicy”, “sweet”, “cozy”, “mystery”, “secret”, “reveal”, “members”, “freebies”, “congrats”, “winners”, “giveaway”, “survey” and names of months (September, October, etc.)

Takeaway: Subscribers like updates, but not daily! 

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: “stock”, “ends”, “chance”, “don’t”, “left”, “miss”, “hours”, “final”, “tonight”, “fast”, “huge”, “tomorrow”, “daily”, “order” and “buy”. 

Takeaway: Subscribers don’t like feeling rushed.

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 of 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.

If a tree falls in a forest and no one is around to hear it, does it make a sound? You spend a lot of time crafting the perfect email, but if no one opens it, they will never know what they are missing.

Take the extra time to write subject lines that will pique your reader's interest. 

• Describe the benefits of your email. What's in it for them?
  
• Keep it fresh. If something works, that doesn't mean you use it every time.
  
• When in doubt, test! MailerLite's A/B Testing feature makes it easy for anyone to test subject lines.

Testing your emails is so important, we wrote a whole chapter on it.

The email subject line and preheader should work together to build anticipation of your valuable content. In our next chapter, we’ll cover everything you need to know about writing effective email content.

P.S. One more thing

Have you considered that your ‘from’ field (the sender) is even more important than your email subject line?

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

Make sure your readers recognize the sender’s name and want to open the email. That means you need to deliver value in every single email to build your reputation.