Guide to writing subject lines for email acquisition

BY Suzanna Chaplin

“I am not a photographer but I can picture me and you together.”

Writing a subject line is akin to a pick up line. It either ends up in a date or a brush off. So no wonder the whole experience is stressful.

For many of us one liners are not our forte and I imagine the stats for how many marriages were a result of catchy one liner are not stacked in favour of the person delivering it! Yet the subject line is just that, a snappy one liner which creates a relationship between you and the recipient. So it’s no wonder that when you write a subject line, questions race through your mind on whether it’s direct enough? Too long? Off-putting? Boring? And that’s even before you thought about whether it will make it through the SPAM filters.

In this article, we have looked at three areas involved in this decision: The length, what is the right incentive and spam words.

Hopefully these tips will help you to have people saying ‘if you were a book, I would be up all night reading you’.


You are often told to keep your subject line short, in order to increase open rates. This may be due to the fact that many email readers and browsers tend to cut off subject lines longer than 50 characters, with some mobile devices only showing 33 characters.
However, against popular belief, Return Path actually found subject lines of 61 to 70 characters had a 17% average read rate — the highest of any length. (Return Path, February 2015). Despite this finding, they concluded, “Our research indicates that there is actually no correlation between the length of a subject line and its read rate. When comparing the number of characters in a subject line to the read rate, the Pearson’s correlation value was -.03, which shows us that there’s no relationship between the number of characters in a subject line and whether or not the email is opened.”
This mirrors our findings; we looked at 402 campaigns sent to 124million across July to September. The campaigns were sent on behalf of 241 brands and sent in the third party. Our findings were below:

No. of Characters Open Rate
0-19 7.59%
20-39 7.35%
40-59 7.99%
60-79 7.53%
80-99 7.04%

As you can see, subject lines which had a length of 40-59 characters had the highest open rate with 80-99 having the lowest. However, the difference in open rates is marginal.


Most mailers agree that offering an incentive increases open rate but which one gets people opening?
We looked at 243 campaigns delivered to 85million people from July to August and segmented the subject lines based on the type of incentive offered and found the following open rates.

Call to Action Open Rate
Competition 9.89%
Offer led 8.97%
Free Guide 8.56%
Free giveaway 8.42%
Comparison 8.17%
Free trial 7.98%
Save Money 7.72%
New Season 7.45%
Money off 7.22%
Price led 6.99%
Percentage off 6.93%
Free Consultation 6.85%
Brand Led 6.82%
Time Sensitive 6.67%
Question 6.43%
Quick and easy 5.99%
Earn money 4.90%
Sale (no percentage off or End date) 3.99%

We segmented out sale messages into those that used ‘Sale’, ‘Percentage Off’ or ‘Time Sensitive’ language. The interesting finding is that when no end date or percentage off is included in the subject line, the open rate is much lower. MailChimp in another test found that subject lines using ‘sale’, tends to trigger spam filters and generate low open rates.

MailChimp also suggested that ‘percent off’ adversely effected open rates. We examined the open rates of the subject lines containing this kind of language and found similar findings to MailChimp. Although, surprisingly, we found that using the word ‘free’ did not negatively affect open rates.

Word Open Rate
Free 7.95%
% 7.11%
Win 6.60%
Percent off 4.80%
! 4.67%
Sale 4.29%

Spam Words:

This leads me onto what is the ultimate list of email SPAM trigger words. Hubspot listed over 100 known spam words to avoid and you could take this and apply it to all your sends but the thing with SPAM filters, they are dynamic. So what is a SPAM word one day, is not always the next day. So it’s not enough to apply a hard list, you need to test your subject line for its SPAM score, every time you send it.
But for those who are time short, here are 10 we have seen this week and just to let you know for the last three years, Your Family, has consistently generated a score of 3.0 – the highest spam score we have ever had for a single phrase!

– Your Family; Spam Score 3.0
– 100% percent guaranteed; Spam Score 2.7
– Act Now: Spam Score 2.4
– Guaranteed; Spam Score 2.4
– CAPITALS; Spam Score 2.4
– Bad Credit; Spam Score 1.7
– Losing pounds; Spam Score 1.6
– Weight loss; Spam Score 1.5
– Percentage off; Spam Score 0.3
– Order now; Spam Score 0.1

Like a first date, we have only scratched the surface when it comes to subject lines and there are many other facets you can discuss and dissect. However, my recommendation would be to test and test some more. What works for one brand, will not work for another. At the end of the day, we didn’t all fall for the same person. So get to know your audience, try humour, incentives, playing around with different words and always test. Remembering what works today, may not work tomorrow. But when you do find a corker, it will be a match made in heaven!


Ready to talk to millions of new customers?