Email Marketing Subject line Best Practices 2021

47% of email recipients open emails based on the subject line.

That’s a pretty powerful statistic. But not unsurprising.

Because in today’s busy world, the inbox is becoming a place that is saturated with email communications. All of which are battling it out to win recipients’ attention.

And with so much noise, the subject line is one of the strongest opportunities for marketers to stand out from the crowd and secure that all-important open.

But even though subject lines are clearly integral to any good email marketing strategy. They are often left until the last minute, quickly typed out as an afterthought just before the email send.

At Pure360, we’re here to help you make the most of your subject lines. Ensuring that you have a solid strategy in place, can write the best subject lines possible, and test for the highest performing results.

For subject lines, better starts here.

Subject line strategy

Before you dive into crafting the perfect subject line, there are some strategic factors that you should consider.

As we’ve covered, a subject line can make or break an email campaign. So it’s important to take a step back and embed some best practice into all of your subject line strategies.


The perfect subject line length is a topic we are often asked about. Unfortunately, there is no magic answer that marketers can adhere to.

In fact, studies have shown that there is little proven correlation between subject line length and read rate. Instead, we would always recommend testing subject line length with your own recipients, which we will cover later in this guide.

But a good rule to follow is to keep subject lines as succinct and concise as possible. In general, waffle and unnecessary information can become distracting or boring to a reader. And as there is only a short amount of space in the inbox to grab a recipient’s attention, marketers should focus on the words that really matter.

Email Marketing Subject line example Waitrose

Character limit

Just to make things complicated, character limits will vary based on device, email client, and browser.

This means that if some subject lines have too many characters, they will be cut off in the inbox. Making it easy for the reader to misinterpret the message.

As a general rule, most email clients will cut off subject lines at around 60 characters on desktop and around 30 characters on mobile. It’s worth considering that 47% of people use mobile to check their emails, a number which is ever-increasing. So you may wish to choose a mobile-first approach when writing subject lines.

Email Marketing Subject line example The Royal Mint

SPAM filters

One of the most significant repercussions of getting a subject line ‘wrong’ is that it can trigger SPAM filters.

This will likely happen if your subject line includes certain words that SPAM filters consider to be red flags. Even for those who have a strong sender reputation.

Fortunately, you can run your email through SPAM checkers which will identify any words that could potentially trigger SPAM filters, and tell you which phrases to avoid.

In the meantime, common SPAM trigger words, phrases, and symbols are:



Act now


Best price

Join millions










By using personalisation in subject lines, brands can increase their open rates by 50%.

Email personalisation increases the relevance of subject lines and encourages recipients to take action. And with the inbox becoming an increasingly busy place, brands should do all they can to stand out from their competitors.

Fortunately, personalisation of subject lines is fairly simple to do.

For instance, brands can utilise merge tags within their ESP to personalise a subject line with the recipient’s first name. This is a simple and easy win.

Or brands can be more sophisticated by segmenting their email campaigns based on persona, demographic, or buying behaviour. And send out subject lines that are most likely to resonate with these segments.

Email Marketing Subject line example Easy Jet

Pre-header text

When writing a subject line, don’t forget about its best friend, the pre-header text.

Pre-header text offers recipients a little more context into your email, further encouraging that all-important open.

And of course, it can complement the subject line perfectly. Brands can use a pre-header text as an extension of their subject line, using it to incorporate less important words and phrases or to elaborate a little more.

Or, they can use pre-header text as a call to action, so that they don’t have to squeeze ‘action’ words into their subject line.

Whatever way you use your pre-header text, ensure that it remains consistent with the language and tone that you have used in your subject line.

Email Marketing Subject line example Look fantastic

Tips for writing subject lines

Now that you have a solid subject line strategy in place, it’s time to get creative with your copy.

Brainstorming ideas for subject lines can feel a little overwhelming to begin with. There are so many options and variables to consider. So, to get you started, here are our top tips for subject line copy.

Use powerful words

There aren’t a lot of characters to play around with within a subject line. So make the ones you do use count.

Choose unique and unusual words that are likely to catch the recipient’s attention, or evoke an emotion. Consider interesting ways of using these words, such as rhyme or alliteration, for added impact.

Email Marketing Subject line example Pour Moi

Give your important words prominence

Recipients will often skim through their inbox, so it’s important to place your most important words in the most prominent position. At the start of your subject line.

This ensures that they won’t be cut off on a mobile device, for instance, and also puts the emphasis on your email subject from the very beginning.

Email Marketing Subject line example Cult Beauty

Ask a genuine question

Questions are a fantastic way to pique interest in a subject line. As long as they’re done properly.

The wrong question can come across as spammy and deceptive. So focus on asking something genuine and thought-provoking. This could include inquiring about the recipient’s thoughts on an important issue or hot topic, for instance.

Email Marketing Subject line example Meet Edgar


Make them relevant

Adding relevancy to a subject line not only makes them more interesting, but it gives them a personalised edge by tapping into a topic that your recipient can likely relate to.

Relevancy could be based on a current holiday, the day of the week, the weather, payday, or other hot topics at the moment, such as a big sports game.

Email Marketing Subject line example Prestige Flowers

Make the recipient feel like a VIP

Everyone loves to feel special. So give recipients the VIP treatment from the very beginning through your subject line.

This could include offering them a discount or gift, inviting them to a private event, or pre-launching exclusive information.

Couple this with some simple personalisation, such as their first name, for some added special treatment.

Email Marketing Subject line example Nasty Gal

Add a sense of urgency

With so many emails in the inbox, it’s easy for recipients to get distracted and never get around to opening your communication.

Encourage the recipient to open straight away by adding urgency to your subject lines. This tactic can increase click-to-open rates by at least 14%.

This could include a reference to a count down, or a limited time period offer.

Best Practice Subject Lines - Offer - The Guardian

Use emojis sparingly

Emojis are a fantastic way to stand out in an inbox full of text. They set the tone of the email and add some colour and visuals to your subject line. However, use them sparingly and use them to complement your subject line. As opposed to replacing words.

Also, bear in mind that operating systems will render slightly different versions of emojis. So you may want to test the variations before sending.

Best Practice Subject Lines - Emoji - Beauty Bay

Have a laugh

There are certain types of emails that everyone gets in their inbox. Sales-focused, delivery notifications, newsletters.

One way to stand out from these emails is to take a more light-hearted approach. Step away from the sales tactics. And have a laugh in your subject lines.

This could be through a joke, a witty line, or even something a little controversial.

Best Practice Subject Lines - Fun - Las Iguanas

Subject line testing

Once you’ve crafted the subject line to beat all subject lines, it’s easy to get carried away and rush into clicking the ‘send’ button. However, it’s important to keep a level head and implement a proper subject line testing process first.

Segment your audience

When it comes to subject lines, unfortunately, one size doesn’t fit all.

That’s why it’s important to segment your email lists, sending out the best subject line to fit specific audiences.

This tactic should be reflected in your subject line testing. Begin by choosing a specific segment of your audience to begin testing on.

And if you haven’t already segmented your lists for your regular email campaigns, this can easily be done using any good AI marketing platform.

Identify elements to test

If you try to test all elements of a subject line at once, it will be difficult to attribute your results to any specific changes.

Instead, aim to test just one element of your subject line at a time. A great place to begin is with one of the most notable changes to a subject line, such as:


Test short and sweet copy against a subject line that is more descriptive.


Do your recipients react better to a question or a statement?


Begin by incorporating the recipient’s first name or business.


Do your recipients engage with emojis, capitalisation, or exclamation marks?

Test a sample size

If your list segment is particularly large, consider reducing this down to test on a sample size instead.

We recommend using A/B testing for subject lines. Any good AI marketing platform will offer this ability. The platform will automatically A/B test your two chosen subject lines, and then will send the highest performing option to the rest of your email list segment.

The benefit of this approach is that the majority of your list will receive the best possible subject line.

An alternative is splitting your list segment 50/50, but in this case half of your list will receive the lesser performing subject line. Resulting in decreased results and potential missed opportunities.

Analyse results

Once your results are in, you can start digging down into the data.

Analyse as many metrics as possible. Open and click-through rates will always be important, but don’t forget metrics such as conversions too.

Pay attention to any interesting trends. For instance, your first subject line may have generated a higher open rate, but your second subject line may have generated more conversions. In this case, your second subject line clearly results in quality engagement, over quantity of engagement.

Never stop testing

One test is never enough. Instead, subject line testing should be an ongoing part of your email strategy.

Focus on testing different elements and different audiences. Even consider using the A/B testing method for every email send to ensure the majority of your list are receiving the best subject line for them.

And of course, audiences changes. Their needs and challenges adapt over time. So subject lines need to continue to adapt too.

Are your subject lines feeling a little lacklustre?

Subject lines are one of the key elements to email success. But while they are only a short line of copy, getting them right can sometimes be a little overwhelming.

Get in touch with our team of experts who can offer you the help, support, and technology to get your subject lines spot on.

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