Musings and ramblings of a digital agency.
By On October 9 2019 Email Marketing With 0 Comments Permalink

The Ultimate Guide to Email Subject Lines

First impressions count. Whether it’s a first date, the first bite of a meal, or the first glimpse of a movie trailer, a good first impression really sets the tone. Let’s be honest: we’re all guilty of occasionally judging a book by it’s cover. That’s why email subject lines are so important. A good email subject line can be the difference between a customer engaging with your content and tuning out completely. 

Check your inbox, and you’ll likely see a variety of approaches to email subject lines. However, they all have one thing in common: striving to make a good first impression.

Of course, this is easier said than done. How many subject lines truly grab your attention? If the answer is ‘not enough’, that’s why we’ve created this ultimate guide to email subject lines. 

Harnessing XCOM’s email expertise, our guide covers the ideal subject line length, the best words and phrases to include in your subject lines, whether to include emojis in subject lines, the power of AI, how to use punctuation & capitalisation in subject lines, and the benefits of originality. 

Subject Line Length

Do you opt for long subject lines that show off your amazing copy, or short subject lines that leave readers wanting more? For email marketing professionals, finding this balance is imperative. Unfortunately, there is no magic formula. The ideal email subject line length can vary depending on your brand, your target audience, the content of your email, and the email client you’re using. Still, there are a number of factors to consider when attempting to craft the perfect subject line. 

Firstly, a high percentage of emails are now opened on mobile devices, which tend to display shorter subject line previews. Between 2011 and 2018, open rates on mobile devices increased by 34%. As of 2018, 61% of users open emails on their mobile devices, with mobile accounting for 46% of all email opens[1]

Additionally, different email clients and web browsers display a different number of subject line characters. Among popular email clients, Gmail displays the first 70 characters of a subject line, while Hotmail/Outlook displays the first 60 characters, and Yahoo Mail only displays the first 46 characters[1]

Further, iPhones display roughly 35-38 characters of a subject line in portrait mode, while Galaxy phones display 33 characters in portrait mode[2]. As such, it is worth performing a thorough analysis of your subscribers’ preferred email clients to optimise subject line length. 

With this in mind, research by Marketo suggests that 7 words (41 characters) is the ideal email subject line length in 2019[3]. This is approximately 10 characters shorter than the average email subject line[1]. 60 characters seems to be the maximum, with an analysis of 1,000 subject lines from top-performing marketers finding that 82% of experts use a subject line of 60 characters or fewer[4].

Marketo’s analysis found that 7-word subject lines produce the highest click to open rate of 10.8%, while 8-word subject lines produce the lowest click to open rate of just 6.6%. 

Even so, subject line length remains a contentious issue, with other studies reaching different conclusions, so it is worth experimenting to determine the ideal subject line length for your audience. Could 7 be your lucky number? 

Buzzword Bingo: Which words work? 

Seven word subject lines may be the sweet spot for email success, but that’s where the real work begins. Which seven words should you use? Again, there is no magic formula to create the perfect subject line. As with everything pertaining to email subject lines, it is critical to find the right message to suit your audience, brand, and industry. Still, there are a few words and phrases you can use to increase your chance of success, and a few phrases you should probably avoid. 

According to an analysis by Campaign Monitor, using the recipient’s first name as the first word of an email subject line leads to the highest increase in open percentages (14.68%)[5]. That’s right, personalisation works. More broadly, personalised subject lines are 26% more likely to be opened than non-personalised subject lines[6]. Other successful words include ‘Invitation’ (9.45% Open%Change), ‘Introducing’ (7.36% Open%Change), and ‘We’ (5.87% Open%Change)[5]

Another analysis by ReturnPath expands on this, splitting subject line keywords into 10 basic ‘types’. The best-performing keyword type was ‘Urgency’ (keywords included expire, expiring, extended, hurry, now, limited time, and last chance), followed by ‘Benefit’ (keywords included best, cheapest, easiest, fastest, prettiest, and quickest), and ‘Command’ (keywords included add, aim, buy, call, click, download, get, open, and try)[7].

At the other end of the spectrum, keywords grouped into the ‘Clickbait’ type, such as ‘get rid of’, ‘secret of’, ‘shocking’, ‘won’t believe’, and ‘what you need to know’, had the most negative impact on subject line performance[7]

Overall, the words and phrases which had the greatest influence on email read rate were ‘still time’ (+15.54% influence on read rate), ‘register’ (+6.70% influence on read rate), and ‘fastest’ (+5.3% influence on read rate). There’s still time: be the fastest to register! 

In contrast, the words and phrases which had the most negative effect on email read rate were ‘secret of’ (-8.69% influence on read rate), ‘2 for 1’ (-6.62% influence on read rate), and ‘running out’ (-3.3% influence on read rate)[7]

There are many other words and phrases that you should probably avoid using in subject lines. This infographic highlights 100 words that decrease email open rates, including common spam phrases such as ‘eliminate debt’, ‘special promotion’, and even ‘this isn’t spam’. 

Including a number in your subject line can also have a positive impact on open rates. Studies have shown that including numbers in blog headlines increases clickthrough rates by 206%[2]. While not wholly comparable, blog headlines and email subject lines serve a similar purpose. An analysis of 115 million emails reinforces this, showing that email open rates increase when a number is present in the subject line[2].

Emojis in Subject Lines: 😊 or 😢?

Are your subject lines 👍 or 👎? If used correctly, emojis can be a powerful marketing tool. Emojis in email subject lines are no exception. According to research by Experian, using emojis in subject lines increases open rates by 56% compared to text only subject lines[8]

Despite this, a study which analysed 2 million subject lines found that only 5% used at least one emoji. Of those that did, 1,600 unique emojis were used. The three most commonly used emojis were a star [⭐], an airplane [✈️], and a red heart [❤️][8]. One explanation for this relatively low emoji usage is that emojis can display incorrectly – or not display at all – in different email and web clients. 

This study also tested the effectiveness of emojis in email subject lines. While the results were “inconclusive”, they concluded that in 60% of tests “the subject line which included an emoji outperformed the same subject line minus the emoji”[9]. Essentially, while emojis may not make a subject line irresistible, they can certainly enhance a well-crafted subject line.

The verdict is in, emojis = ❤️. 

The Power of AI

These days, subject lines aren’t exclusively the domain of humans. AI-generated email subject lines are becoming increasingly common (there’s a good chance some have already found their way to your inbox!). Companies such as Phrasee can generate AI subject lines that perform better than 98% of human-written subject lines[10]

It’s worth considering if AI-generated subject lines are a good fit for your brand. For a deeper dive into the role of AI in email marketing, check out our blog post here


Punctuation is powerful! Whether you’re conveying excitement, uncertainty, or just ensuring readers can actually understand your sentence, punctuation is an important tool in any writer’s arsenal. Punctuation is an equally powerful tool when it comes to email subject lines. However, the rules are slightly different. 

An analysis of over 650,000 subject lines showed that punctuation is most effective when used sparingly. For example, Touchstone found full stops should only be used in 2-4% of your subject lines. Doing so results in a 10-20% increase in open rates, as well as an 0.5% increase in deliverability. In contrast, overusing full stops can reduce open rates by 5-10%[11]

A similar trend is evident with exclamation marks. Imagine receiving an email with the subject line: “Special Offer – Don’t Miss Out!!!!!!!”. Block. Mark as spam. Unsubscribe. Again, an overuse of exclamation marks can be extremely detrimental, whereas a well placed exclamation mark can produce positive results. Touchstone’s analysis found that 70% of brands that use exclamation marks “sparingly” experience a 10-20% increase in open rates[11]

On the other hand, it might be better to avoid question marks entirely. Why? 72% of brands see lower click rates when including a question mark in their subject line. On average, click rates decrease by 8.1% for subject lines that include a question mark compared to all other subject lines[11]

Overusing punctuation may be bad, but misusing punctuation is arguably even worse. Including spelling or grammar errors in your subject line is a definite ‘no’, so ensure you thoroughly proofread your subject lines before hitting send. 


Do you want to whisper your message, or SHOUT it? Capitalisation is another important consideration when writing email subject lines. 

There are three distinct formulas for capitalisation in email subject lines: sentence-case (the first letter of the first word is capitalised), title-case (the first letter of every word is capitalised, except for articles such as ‘a’, ‘the’, and ‘an’), and lowercase (every letter is lowercase). 

According to AWeber, 60% of subject lines use sentence-case capitalisation, 34% use title-case capitalisation, and only 6% use all lowercase. While title-case is the most common form of subject line capitalisation, it could be worth experimenting with lowercase, with some experts reporting “~80% open rates and ~30% click rates” thanks to this strategy[4]

Be Original! 

How many times have you received a slight variation of the same-old, cliched subject lines? An analysis of over 21 billion subject lines deemed 71.5% of subject lines ‘unoriginal’, meaning only 28.5% of subject lines are considered ‘original’. For this study, ‘original’ was defined as “a subject line that contained at least one word the sender had never used before”[12]

More importantly, original subject lines outperform unoriginal subject lines. Original subject lines enjoy a 2.77% unique click rate compared to just 2.07% for unoriginal subject lines. Likewise, original subject lines receive a 15.79% unique open rate, while unoriginal subject lines have a unique open rate of 14.65%[12].

The message is clear: get those creative juices flowing and make sure your subject lines are original! 

Test, Test, Test 

Despite all our tips, subject lines can still be a guessing game. It’s possible to write what you think is a perfect subject line – the ideal length, with a healthy dash of personalisation, well-placed punctuation and capitalisation, and masterful use of emojis – and still not get the engagement you were hoping for.

That’s why the most important tip of all is to test your email subject lines. The most reliable way to discover what’s working, what’s not, and how to improve is to consistently A/B test your email subject lines. 

You can test a variety of factors, including short vs long subject lines, which words increase engagement and which drive customers crazy, and how your audience reacts to emojis. 

Make sure you’re clear on what key metrics you’re looking for in your tests, as well as what defines a ‘successful’ subject line for your brand. 

Want your subject lines to be the subject of discussion? Contact XCOM today to find out more about all things email! 

Contact us














Image by: Muhammad Ribkhan from Pixabay