The Art, Science and Future of Subject Lines

Subject Line


Subject lines matter

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

They can make or break the success of a campaign. The greatest email in the world is doomed to fail if nobody bothers to open it.

Your subject line could be the last impression you get to make to a recipient, as 69% of email recipients report email as spam based solely on the subject line.

Leaving your subject line to chance is a risk that marketers can’t afford to take and unfortunately, there is no simple rule to subject line success. But the good news is that subject line mastery can be taught.

We believe the humble subject is more important than ever. Why? Because GDPR has heightened the need to ensure our email communications are as relevant and valuable as possible. And also because of new technological advancements — more on that later.

Perhaps the most important reason is due to the sheer amount of noise in the average inbox. Globally, each email address receives an average of 77 emails every day. That’s a lot of subject lines to compete with.

“Average open rates are between 15% and 25% range, depending on industry, so for the 75% to 85% of recipients who do not open any given email they receive, the subject line IS the email”

– Dela Quist, Founder & CEO – Touchstone Intelligent Marketing | CEO – Alchemy Worx

Your subject line may be your only chance to get your message in front of your audience, so it’s worth spending some time getting it right.

In this guide, we’ll be exploring the art and science of the subject line. Read on to learn how to become a subject line master and deliver record-breaking open rates.

Subject line best practices

Before we dive into the psychology behind irresistible subject lines, it’s important to start with the basics.

Get the basics wrong and your subject line may not appear as intended, or worse yet, it may cause your email to land straight in the SPAM box.

Subject line length

Subject line length is one of the most commonly researched topics in email marketing.

The general consensus is that shorter subject lines work best, but before you start writing short and snappy subjects lines, there are two factors to consider:

1. How many characters are typically displayed on each device and mail client

2. How length influences the open rates with your audience

Character limits by device and mail client

Every mail client has its own character limits for subject lines. This will also vary depending on the device, and also the browser width on desktop.

Longer subjects lines may end up being truncated in the inbox. And truncated subject lines may end up causing some mild embarrassment if certain words are cut off.

‘Look’ could become ‘Loo…’

‘Tartan’ could become ‘Tart…’

‘Analysis’ could become — ahem — you get the idea.

Most mail clients will truncate subject lines at around 60 characters on desktop and 25-30 on mobile. Considering that more and more emails are opened on mobile with each passing year, it’s wise to use shorter subject lines where possible. We recommend sticking to under 50 characters to be safe.

If you are struggling to get your message across in fewer characters, consider using the preview text to include key information — more on that shortly.


Subject lines length and engagement

The truth is, there is no magic subject line length that consistently drives better engagement.


A study by Return Path showed there was very little correlation between length and average read rate. In fact, the most commonly used subject line length (41-50 characters) had the third lowest read rate.

A good rule of thumb when writing your subject line is to keep it as concise as it needs to be, but don’t be afraid of testing longer subject lines.

You can also look at your own data from your previous email sends to see if you determine a correlation between subject line length and open rates.

That said, even if you were to find a sweet spot, that doesn’t mean it’ll always be right.

The type of campaign, the context of the email, and who you are sending to will all be important in determining what content needs to go into your subject line.

For example, a sale announcement might only need a short subject line to grab the recipient’s attention. While an transactional email might include the product name, order number and expected delivery date would result in a longer subject line.

Avoiding SPAM filters

Like all your email content, subject lines are subject to SPAM filters. A subject line containing a SPAM trigger word may skip the inbox and end up in the SPAM box. The same goes for special characters or sudden capitalisation of words.

Common SPAM trigger words and phrases include:

Call free
Call now!
Free offer
Free quote
Money back
No fees
Risk free
Offer expires

A strong sender reputation could allow you to use these phrases and not be detected as SPAM, but even if you get past SPAM filters, these phrases may be red flags for the recipient due to their association with spammy and deceptive emails.


Personalising email subject lines can increase open rates by 50%.

There’s no question around whether personalisation is worth the effort. But the ultimate key to success is understanding how to use it in innovative ways.

Benefits of personalising subject lines

1. Personalisation grabs the reader’s attention

There is scientific research that suggests that hearing one’s own name has a huge impact brain activity. Even in a crowded room with lots of noise, our ears will prick up if we overhear someone saying our name.

Using someone’s first name in the subject line could have a similar effect. Their name will catch the eye as they skim.

Considering the sheer volume of emails that bombard our inbox, any tactics that encourage a recipient to give your subject line a second look are worth exploring.

2. Personalisation increases relevancy

That said, our own research revealed that only 8% of people say they engage with emails that address them by their first name.

So whilst using personal details can grab their attention, you also need to be able to demonstrate relevance and value.

Location is a good example of how you can show your relevancy without simply showing off the data your hold on each person. For example, a travel brand could highlight holiday deals departing from their nearest airport in the subject line.

3. Personalisation encourages action

Using personalisation to reflect a customer’s behaviour is a highly effective way to encourage engagement. It gives recipients context about why you’re contacting them, what you want them to do next, and ultimately, why they should care.

Take, for example, an abandoned cart email. A generic subject line might read:

‘Did you forget something? There is an item still in your basket’

With personalisation, you can get much more specific — such as highlighting the product name, low stock levels, or recent price changes.

‘Sarah, were you still interested in our ‘Rose Gold Earrings’? Only 3 left in stock.’

Types of personalisation to use in subject lines:


Josie, our winter sale has landed

Is Fido running low on Adult Dry Dog Food?

Booking reminder for Mr David Bannister


The best places to eat in Bristol

View the latest house price trends in BN3

Latest holiday offers flying from Gatwick Airport

Product names

Complete the look: Five items that will go great with your new Mulberry Clutch

Is it time to stock up on Whey Protein Powder?

Leave a review of ‘Red Dead Redemption 2 ’


Your membership is set to expire on 05/02/2019

Just 4 days left until your membership expires

You delivery will arrive on Wednesday 17th April – will you be in?


14 new Teaching Assistant jobs in Bristol

Check out our online Marketing courses

Just released: Salary benchmarks for Marketing Managers


Here’s what’s new in menswear

The best hair products for curly hair

5 holidays destinations for families with young children


See where your 22,000 air miles could take you

You saved £30.29 on cashback last month

Save up to £73.22 on your energy bill – view your top quotes

Optimising beyond the subject line

While the subject line will undoubtedly be a big factor in whether people open your email, it’s not the only element you need to consider.

From Name

The ‘from name’ is actually the first piece of information subscribers will see in most mail clients. It tells the recipient who has sent the email.

B2C companies are more likely to use their brand name. While B2B companies might choose to have emails appear to be sent by an individual, however there’s no wrong or right way to go about this.

For example, you might find that a combination of an individual and brand name works best.


Or you might want to use different ‘from names’ to indicate the different types of emails you send.


Some mail clients will cut off ‘from names’ after a certain number of characters. This may vary based on whether it’s opened on mobile or desktop, and even based on the browser size.

For example, Gmail might cut off a ‘from name’ after 20 characters, so keeping it short is recommended.

Top tips for your from name:

– There’s nothing wrong with using your brand name — don’t underestimate the power of brand familiarity.
– Avoid using an unwelcoming ‘from name’ such as
– Test your ‘from name’ across different mail clients and on mobile.
– A/B test your ‘from name’ to see what drives the best result.

Preview Text

The preview text is what appears after your subject line. It’s exactly what the name suggests, a preview of the very first text that appears in the email.

For the recipient, it gives extra information about the content of the email.

For the marketer, it’s an extra opportunity to grab your recipient’s attention and entice them to open.

Wait, isn’t it called the pre-header text?

No, the preview text is what the mail client serves up in the inbox.

The pre-header text is what you can use to control the preview text. As the preview text takes the very first text in an email — including image alt tags — pre-header allows you to dictate what is shown after your subject line.

Pre-header text can be displayed or hidden before the main email copy. It gives you an opportunity to play off your subject line, give extra information and add more personalisation.

The vast majority of mail clients support preview text. Depending on the length of your subject line, they may offer you a greater number of characters to get your message across in the inbox.

Think of your subject line and preview text as a partnership. They’re a one-two punch that will help you cut through the noise and entice your recipient to open.

Top tips for your preview text:

– Avoid simply repeating what the subject line says.
– Complement your subject line. If your subject is short and intriguing, add information that builds on that intrigue.
– Use personalisation to make it feel relevant.
– Use a call to action that drives urgency. Your subject line might have grabbed their attention, but you still need to compel them to click.
– Use a conversational tone — after all, this is meant to be a preview of the email content.

Measuring subject line success

How do we measure the success of a subject line?

Open rates, right? Well, yes and no.

Yes, the open rate is the most commonly used metric to measure subject line success. After all, it plays a huge part in influencing a recipient to open the email.

But only focusing on open rates is risky. I’m sure everyone has opened an email only to find the subject line had little to do with the content of the email. Deceptive tactics may boost your open rates, but they leave a bitter taste and could result in someone unsubscribing.

Looking at your open rate in relation to other metrics will give greater context as to the success of your subject line.

A low open rate paired with a high click-to-open rate might suggest your email content was great, but your subject line needed work. Or it might suggest that your subject line was perfect for driving highly qualified traffic to your website.

It really depends on the overall goal of your email. Your subject line should be a reflection of that goal.

If you’re emailing to let subscribers know that your seasonal sale is ending on your website, you’re going to want to measure the click-through rate in addition to the open rate.

However, just measuring the open rate on an order confirmation email might be fine as all the important information could be contained within the email and would require no further action.

Then there is what Dela Quist calls the ‘halo effect’ of subject lines. The ‘halo effect’ suggests that even an email that doesn’t get opened might influence a recipient positively.

For example:

1. A person skims through their inbox
2. They see a subject line from a brand announcing a sale but don’t open the email
3. Later on, they visit the brand’s website via a Google search and make a purchase in the sale.

This type of influenced attribution is somewhat difficult to measure, but it’s worth bearing in mind that email isn’t just a direct response channel.  Email can also be a valuable channel for driving continued brand awareness, much like TV and print advertising.

Testing your way to success

Best practices are rife in email marketing for good reason — there are many technical aspects that define whether your email even lands in an inbox.

We believe it’s important to know what best practice looks like, but don’t let it stand in the way of experimentation and testing.

Here’s a step-by-step guide to implementing a subject line testing strategy:

1. Set a measurable objective

First up, set up a measurable objective. The most common metric to measure is the open rate, but you also might consider testing your subject line to improve click-through rates and conversions, or reduce unsubscribe rates.

2. Create SMART goals

Once you’ve chosen your metric, it’s time to decide what you want to achieve with your testing strategy.

Following the SMART goal methodology lets you set targets that are:

– Specific
– Measurable
– Achievable
– Relevant
– Time-bound.

For example:

Increase average open rates rate from 12 to 20 percent by the end of 2019.

3. Create your hypotheses

A hypotheses will ensure your testing strategy is informed by an understanding of your audience rather than just changing a random variable.

A good hypothesis includes:

– What you are going to change
– What you think the outcome will be
– Why you think that will be the case

For example:

‘Using shorter subject lines will increase open rates as our customers typically read our emails on their mobile’.

4. Pick your variable

In the above example, the variable to test will be subject line length but there many variables available to you.

For example:

Personalised vs generic

‘Sarah, our summer sale has landed’ vs ‘Our summer sale has landed’

Sentence order

‘20% Off Trainers’ vs ‘Trainers 20% Off’


‘5 places to visit before you turn 30’  vs ‘5 Places to Visit Before You Turn 30’

Tone of voice

‘Sale ends soon – don’t miss out on savings’ vs ‘Sale ends soon – you’d be a fool to miss out’

Asking a question vs making a statement

‘How do your open rates fair against industry benchmarks?’ Vs ‘Measure your open rates against industry benchmarks’.

It’s important to restrict your test to a single variable. Introducing multiple variables will make it hard to pinpoint what actually made a difference.

5. Determine your sample size

Now you need to consider the sample size needed to get meaningful results from you test. Testing on too small a sample size won’t tell you a great deal.

You’ll likely need to test your variable over multiple campaigns to find a meaningful answer to your hypothesis.

Here’s a handy calculator to help you work out what sample size to use.

6. Run your A/B tests

A subject line selector, like the one available in PureCampaign, will allow you to test two or more subject lines against each each other.

Once your email is sent, you’ll get notified about which variation performed best and whether a winner could be declared by statistical significance.

If this feature isn’t available in your email service provider, you can also run an A/B test by splitting your list. However, you’ll need to ensure you split your list randomly to reduce the chance of bias.

7. Collect and analyse the results

Document the result of every test you run, being sure to note down what variable was tested. Bringing your test data into a spreadsheet will allow you to analyse multiple campaigns.

It will also allow you to look back over previous tests to inform new hypotheses.

8. Draw your conclusion

Once you have conclusive results, you’ll be able prove or disprove your hypothesis.

Sometimes your test may not have the desired result. Performance could go down, but don’t feel disheartened. A failure to prove a hypothesis should be seen as a success.

If no statistical significance is found, consider adapting your variable and running the test again and it goes without saying that you should be constantly testing.

The psychology of successful subject lines

Psychology and marketing go hand-in-hand.

At its core, marketing is about influencing perceptions and behaviours. So understanding the psychology behind why people think and act in certain ways is undoubtedly beneficial to marketing.

The primary goal of a subject line is to encourage the reader to take action — open the email (well duh). It’s the perfect playing field for the marketer to use psychology to influence that action.

Consumer psychology is a broad subject that has been researched and explored for decades, if you were to read only one book on the topic, we recommend Dr. Robert Cialdini’s Influence: The Psychology of Persuasion.

In the book, Cialdini outlines ‘6 Principles of Influence’. We’ll be explaining what they are and how they can be used in creating subject lines that influence action.

1.   Reciprocating

People feel the need repay anything given to us.

In the 80’s, the Hare Krishna movement used recriptation to encourage donations by offering flowers to people at airports. They found that people who had received a flower were much more likely to make a donation than those who didn’t, despite the fact that most of the time the flowers were immediately binned.

There are plenty of ways to use reciprocity in your subjects lines.

Offering a free gift to subscribers is great way to encourage action. Many subscription services like Dollar Shave Club will offer their first box for free.

The same goes for offering discounts. You’d be a fool not to highlight the savings customers can make in your latest sale. But you should also think about how discounting can feel more personal to the individual, such as rewarding loyalty with money off.


– A free gift is waiting for you
– It’s about time we treated YOU
– Bag a free gift with every purchase this weekend
– You made the cut. We’ve giving 30% off to our most loyal customers
– Your just two nights away from a free reward night

2. Scarcity

People place more value on things when they are less available.

We’re a fickle bunch. As soon as we feel like we can’t have something, we want it.

Back in 2003, when British Airways announced that its Concorde service would be coming to an end, sales went through the roof as people scrambled to secure a seat on one of the final flights.

Scarcity is perfect for creating FOMO in retail when stock levels are running low. Similarly, travel brands can create urgency when hotel or flight availability is short supply.

You can also use time-related scarcity to highlight the end of sales.


– Only 3 left in your size. Grab a pair of Adidas Ultraboost before they go
– High demand for the hotel you browsed – only 4 rooms left on our site!
– Sale ends Friday – get 30% off before prices go up
– Your air miles are about to expire
– 4 days left on your valuation of £13,710 for your KIA SPORTAGE

3. Authority

People follow the lead of credible, knowledgeable experts.

Building your brand up as an expert in your field is good marketing. If you do something well, make sure your audience knows about it.

You can also refer to endorsements from other experts to build your authority.

Authority ultimately comes down to trust. Using lots of exclamation marks or getting creative with CaPiTiLaSaTiOn might catch the eye, but it could hurt your credibility and your open rates.


– Our stylists pick their top fashion trends for Summer 2019
– The UK’s best broadband provider – Which?
– Our staff choose their top 10 novels of 2019
– Up-and-coming travel destinations for 2019 as chosen by our experts
– Five CEOs reveal the books that changed their lives

4. Consistency

People like to be consistent with the things they have previously said or done.

When we make an initial commitment, we’re more likely to make a similar commitment at a later date.

Additionally, we also we want to feel like we made the right choice when looking back at previous decisions we’ve made. Cialdini cites a study where people placing bets on horses felt more confident on their chances of winning after placing bets, rather than just before.

Any company that relies on repeat business should look to use consistency in their marketing. Charities looking to drive donations could refer to the previous donations a person made in a subject line.


– Will you donate again?
– You helped us raise £120,000 for cancer research – can you help again?
– Complete the look – these items will go great with your ‘biker jacket’
– Upgrade your membership to unlock twice as much cashback
– Time to top up. Are you running low on ‘whey protein’?

5. Liking

People prefer to say ‘Yes’ to those that they like.

We are more influenced by people we like — that could be a close friend, a celebrity we admire, or a friendly salesperson who pays us a compliment.

Cialdini lists some factors that cause the liking rule to take effect:

1. Psychical attractiveness: we assign favourable traits to good-looking individuals
2. Similarity: we like people who look similar to us or have similar personality traits or interests
3. Compliments: we like people who pay us compliments even when it might be false
4. Cooperation: we like people who are working towards a goal that will have a mutual benefit
5. Association: We like things associated with people we like — hence why good-looking models are commonly used to advertise products

While some of this may seem shallow, it still remains important for your marketing to create positive brand associations.

So, how could this play out in subject lines?

A good start would be to use the same language as your audience. Speaking their language will encourage customers to see the similarities between themselves and your brand.

Asos is a good example of a brand that uses the same language as its customer base. The brand is chatty, informal and isn’t shy of using slang.

Don’t be afraid to let your brand personality shine through in your subject lines.


– Payday + Us = Trouble!
– Treat Yo Self – Grab a bargain in our summer sale
– Your weekly recommendations handpicked just for you
– Here’s how we get over the holiday blues
– Sarah, we think you’ll love these new releases

6. Social proof

People will look to the actions and behaviors of others to determine their own.

Social proof is an interesting psychological phenomenon that all marketers should familiarise themselves with.

Cialdini ran a study to figure out the best strategy for getting hotel guests to reuse towels.

The study involved testing different signs in hotel rooms each of which encouraged the guests to reuse their towels to help the environment.

The first sign was a generic ask.

Help save the environment

You can show your respect for nature and help save the environment by reusing your towels during your stay.

The second sign introduced the idea that other guests were already playing their part.

Join your fellow guests in helping to save the environment

Almost 75% of guests who are asked to participate in our new resource savings program do help by using their towels more than once. You can join your fellow guests to help save the environment by reusing your towels during your stay.

This sign resulted in 26% more guests reusing their towel.

A third sign was also tested. It had a similar message to the second sign but referenced the room to make it even more specific to the individual’s situation:

75% percent of people who have stayed in this room have reused their towel.

This sign increased reusage by 33% when compared to the generic message.

There are plenty of ways you can use social proof in subject lines. You could highlight your best selling products, refer to positive customer reviews or establish a social norm that your audience will be compelled to follow.

You can also layer social proof on top of scarcity techniques to create urgency.


– 20 people recently booked the hotel you were interested in
– Are You Ready to Join 5,000+ Marketers for the Event of the Year?
– 35,000 people are already benefiting from our meal plans
– Shop our most popular pieces of the year
– The top 5 beach destinations chosen by people like you

Subject line tactics

Knowing more about what influences behaviour is one thing, but you still need to put it into action. Additionally, Cialdini’s principles of influence are not the be all and end of when it comes to writing great subject lines.

Here are tactics you can start using;

Be concise

Being short and sweet can help differentiate between the longer subject lines that commonly fill our inboxes.

The wait is over

Act quick


Spark curiosity

Humans have a natural desire for closure. Create curiosity with a vague or intriguing subject line will play to this desire.

Avoid these marketing mistakes

The secret to a better night’s sleep

We made a thing

Make a list

Listicles have been at the forefront of content marketing for a while now, and they’re also highly effective in subject lines.

10 timeless pieces that should be in every wardrobe

8 emerging travel destinations you have to visit NOW

Three proven ways to be more productive

Get statistical

People love a good statistic, especially when it challenges or validates our way of thinking.

Cart abandonment is costing UK retailers £18 billion every year

35% of searches by 2020 will be voice

75% of employers want candidates with this skill

Ask a question

Asking a question instantly encourages the reader to create an internal dialogue. But be mindful not to ask questions that can easily be answered with “no”.

Could you be making these hiring mistakes?

What’s your dream holiday?

How important is your smartphone battery life?

Be controversial

Being controversial is highly effective when done right, but it could alienate or even offend if done in heavy-handed way. It comes down to knowing your audience.

You’re doing these exercise wrong

Nobody is reading your emails – here’s why

5 fashion trends that need to get in the sea

Make them chuckle

Creating a truly funny subject line is easier said than done. But even if your joke doesn’t land, it could still grab your reader’s attention. Bonus points for using a creative pun.

HIIT me with your best squat

Swipe right on these latest deals

The cat’s meow. Check out our leopard print collection

Use keywords

It’s unlikely that recipients will fully read the subject line of every email they receive, so including keywords will help grab the attention as they skim. They also make your emails more searchable.

Summer Dresses – Explore our new arrivals

[Webinar] How to Master Marketing Automation

Save 20% off Nike, Adidas and New Balance Sneakers

Command an action

Sometimes it doesn’t hurt to tell your read exactly what you want them to do. Using strong command verbs turns your subject line into its own call to action.

Use them or lose them: your air miles are expiring

You need this bag in your life

Fill your wardrobe with our latest collection

The future of the subject line

The subject line will continue to play an important role in email marketing, and new technology will mean marketers need to be ready to adapt and optimise their approach.

In this section, we cast our eye into the not too distant future and the new advancements and trends that will impact the subject line.


An estimated 141 million smartwatches were sold in 2018, taking the overall estimated sales to nearly 300 million units.

That’s a lot of inboxes wrapped around a lot of wrists.

Yet, there hasn’t been too much focus on email marketing strategies for smartwatches despite the fact that 25% of owners use their device to check their emails everyday.


Small smartwatch screens will force marketers to get creative about how they optimise subject lines. The Apple Watch truncates subject lines at around 16 characters which makes brevity key to success. We see a growing trend of short and sweet subject lines to enable optimisation across as many devices as possible.

Voice and smart speakers

1 in 10 UK homes own a smart speaker and that number looks set to rise over the next few years.

There’s been some speculation about how voice might impact email marketing strategies but until recently there wasn’t a huge amount of support from smart speakers. Amazon Echo only added email reading capabilities back in late 2018 and currently only Gmail,, Hotmail and are supported.

Functionality is limited too. Amazon Echo can do the following with your email:

1. Read it
2. Reply to it
3. Delete it
4. Archive it
5. Skip to the next message

However smart speakers and voice assistants are relatively immature technology and further mail client support and functionality is inevitable.

So how could voice impact your subject lines?

Smart speakers turn emails from a visual to an audio experience. If anything, voice plays to emails existing strengths of being personal and adaptable.

Email marketers have long been tasked with standing out in busy inboxes through the creative use of language.

In text form, a subject line can get away with being condensed. The following subject lines are fine in text form, but might sound disjointed when read aloud by a voice assistant.

[Report] Top 100 Fastest Growing Brands

Ugh I hate Janua-WAIT EXTRA 30% OFF SALE?!

Plus you’re oh so witty pun may fall on deaf ears when read aloud!

Write up your street

Flour power

Voice will require more conversational language and might even require longer subject lines.

Artificial Intelligence

The biggest new development to act on now is Artificial Intelligence (AI).

At Pure360, we’re big believers in the potential of AI in email marketing, which is why we’ve partnered with Phrasee — a marketing platform that uses AI to write better subject lines than humans.

Phrasee is capable of generating millions of on-brand, human-sounding subject line variants at the touch of a button. What’s more, it focuses on generating subject lines that drives more opens and clicks.

And it works.

Find out more about our partnership with Phrasee.

The future of AI in email marketing is full of exciting possibilities.

We anticipate AI being used to generate highly personalised subject lines based on its ability to profile each individual customer. And it won’t stop at subject lines. AI has the potential to write entire email copy, manage content production, optimise call to actions, and even control your entire campaign schedule.


The subject line is not a shiny new part of your email program, but it still deserves a lot of your attention.

Inboxes will continue to stay noisy, so differentiation is key to success.

New technology will put further constraints on subject lines, but it will also present opportunities to innovate.

And ultimately, a deep understanding of your audience will be your biggest driver of success.

We hope this guide has motivated you to take a closer look at your subject lines and inspired new ideas to implement and test.

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