Home Resources Blog

While there’s no silver bullet when it comes to creating a sticky subject line, it doesn’t hurt to turn to the data scientists for a bit of advice. Thanks to the painstakingly-detailed “Subject Line Analysis Report” from Adestra, light has been thrown on some of the most commonly-used words and phrases used, alongside what influence they have on opens, clicks and unsubscribes.

Assessing the impact of words like “free”, “limited time” or “register” in subject lines is undoubtedly a tricky proposition, given that they are just one of a squillion variables that contribute to how an individual campaign performs. Given this trickiness, we couldn’t help but have our curiosity piqued by the extensive subject line analysis presented in Adestra’s report.

In this post, we’ve picked out some of the most interesting findings. Keeping in mind that your mileage may vary from what’s presented in the report, let’s look at some of the best and worst performing keywords.

“Free Delivery”… But with a catch

As they say frequently in some parts of the world, “freedom isn’t free.” This applies loosely to subject lines, too – according to Adestra’s report, featuring the word “free” in subject lines results in a -3% drop in opens in comparison to the average amongst all email campaigns surveyed. However, the phrase, “free delivery” provides a 50% uplift… But also results in an 82.4% lift in unsubscribes, too. A similar trend can be found in subject lines containing “voucher” (20% lift in opens, 60.6% more unsubscribes) and “sale” (23.2% lift in opens, 31.6% more unsubscribes) – my personal assumption here is that subscribers are initially attracted to emails that appear to be containing a offer, but may be more likely to react by unsubscribing if they find their assumptions or desires to be left unfulfilled. One to discuss in the comments, no doubt.

On the other end of the spectrum, “report” is a real downer, resulting in a 23.7% decrease in opens). Similarly, “learn” also apparently sounds like a lot of hard work for many readers, averaging a 35.5% drop. While it might be tempting to look at the positively better unsubscribe rates for campaigns using these words, it’s worth noting that if a subscriber doesn’t bother opening a campaign, they’re not going to give themselves the chance to unsubscribe, either (assuming this wasn’t taken into account in the numbers).

When to send: Daily, weekly or monthly?

Another interesting finding is that subject lines containing the words, “daily”, “weekly” and “monthly” all perform differently – most likely, because this reflects their actual send frequency. In Adestra’s words:

There is also a strong logic in creating an expectation to receive scheduled messages. Communications that are sent out “Daily” and “Weekly” perform strongly. This is because your customers begin to expect your emails on a frequent basis, and get into the habit of reading them. At the other end of the spectrum, “Monthly” is probably too infrequent, and you’re losing the top-of-mind position for which you are striving.

While this isn’t directly correlated to language use in subject lines, it’s an interesting observation… Especially to us, as we send a monthly newsletter – which you should subscribe to!

Of course, the small print…

Adestra’s research is based on results from 90,000 email campaigns, each which have been sent to 5,000 or more subscribers – with impact being measured as the difference between the average open rate of emails with a subject line containing, say, the word “sale” and the industry average open rate. Same goes for their figures on clicks and unsubscribes.

Something to note is that variation between open rates may not amount to as significant a change as may initially seem – for example, if the industry average open rate is 20%, a 12% lift on this will result in an actual open rate of 22.4%. Of course, if you send to a large list, a 2-3% lift in open rates may make a big difference to your overall results.

Many thanks to Adestra for this painstaking work – you can download their report for full results (requires signup). Now, over to you – do you have a word, or phrase that works particularly well in your subject lines? Have you run an interesting A/B test on a recent campaign? Let’s share notes in the comments below.

  • Christian Milz

    Very interesting topic here. I actually have a few comments:

    – We have clearly seen that personalized subject lines perform quite better when it comes to OR and also have a positive impact on CTR and Unsubscribe Rate. So including the customers name helps us a lot

    – The shorter and clearer we keep our subject line the better it performs

    – Quite interesting to me is that using prices, discounts, the word free, offer, etc. in subject lines doesn’t lead to a lower delivery rate anymore due to spam. I remember the times when those keywords were very critical

    All in all we have seen very good results with subject lines that give a clear impression on whats gonna be seen in the Email and it didn’t result in high Unsubscribe rates.

    Last but not least, the Email design itself has nowadays a huge influence on the OR -> Same subject line but much nicer design brought us an huge increase in OR

  • Andy Francis

    Interesting reading but I do think a lot of it seems fairly obvious to anyone that’s been email marketing for a decent length of time.

    What we found far more useful and effective was a study on subject line length. We compared subject line success across all our clients (Jan-Jun 2013 and exclusively e-commerce market) and found quite a stark contrast between those of 4-6 words and those that were 6+ words across all three KPIs which were OR, CTR and the biggy of actual SALES.

    Then you add in the work Adestra have done on keywords then together you have the key to the holy grail of the perfect subject line right? :)

    ok maybe not that simple but we find the best rules to apply are:

    1. Don’t lie or try to fool anyone
    2. Would you open it?
    3. Keep it short and to the point

    Oh did I mention that England have the greatest cricket team in the world?

    Greeting from Ashes central….


  • Balint

    Hey Christian,

    I’m a bit confused. How did a design change brought you increase in open rates?
    People open the email and then see the design. What am I missing here?

  • supernath

    Hi Balint,

    I’d guess a lot of people opened with images off and then opened the next email, which wouldn’t count it as an open.

    If the design looks good, people are more likely to download the images to see what it is about, which counts it as an open.

  • Balint

    hmm thats a good explanation. although the design of the email without images has to be pretty slick to get that effect, I imagine. Still I think without images its still the content that would make people turn on the images. (and thus register the open)

  • Parry Malm

    Just a point on subject line length (full disclosure – I’m the author & statistician behind Adestra’s report.)

    I looked at subject line length to a great extent and while there does appear to be a correlation between length and OR, the short run variance is wildly vacillating. Therefore I’d not put all your eggs in the “short subject line” basket.

    My (albeit not tested) theory is that if you’re going for a direct response message (Sale on now!) then go short. If you’re looking to generate awareness and brand top-of-mind, then go long and embed multiple benefits in the subject line.

    And I guess I’m a bit of a cynic when it comes to design – if the content is crap, no matter how pretty it is, you won’t get any sort of CTR off the back of it. Focus on distributing killer content and the rest will follow naturally.

  • Ros Hodgekiss

    Hi Parry, it’s great to have you here! Really appreciate you chipping in on subject line length and wording – and of course, sharing your research with us. Agree – focusing on your content is where it’s at, but sometimes, we really need a solid reminder!

  • Shana

    I just posted on my own blog recently about current trends I’ve been seeing. Numbers in context seem to create some compelling results ie “5 ways to… ” – not a serious study just my personal observations based on the results I see. You can see more here http://emailchic.com/post/59619227588/10-tips-on-how-to-craft-a-dynamic-subject-line-ala – really not trolling to post my stuff, just happens to relate.

  • Ros Hodgekiss

    Hi Shana, thank you so much for sharing your blog post, it certainly is on topic, so glad to see it here. I really like the “DIY” observation – the best way to get people interested is to not just firehose them with information, but show them how they can apply themselves, too. A great reminder for everyone :)

  • Angus Peacocke

    We are a small news / publishing business and in the old days we would have called the subject line “the slug”.

    Not really sure why, but if your slug was no good the editors wouldn’t look at your story in the Q of available stories for publishing. (just like your subject line).

    So the slug is basically a headline – as is your subject line.

    There is a craft to good headlines – but basically they need to be short crisp and to the point.

    If you want to increase your open rate – make sure you don’t give the story away in your subject line.

    ie if your offering a shoe discount – leave the quantum of that discount out of the headline (subject line).

    Tweaking this stuff makes a huge difference to opening rates and click through rates.

    We have developed a whole new way of presenting our headlines in our enewsletter to reflect it.

This blog provides general information and discussion about email marketing and related subjects. The content provided in this blog ("Content”), should not be construed as and is not intended to constitute financial, legal or tax advice. You should seek the advice of professionals prior to acting upon any information contained in the Content. All Content is provided strictly “as is” and we make no warranty or representation of any kind regarding the Content.
Straight to your inbox

Get the best email and digital marketing content delivered.

Join 250,000 in-the-know marketers and get the latest marketing tips, tactics, and news right in your inbox.


See why 200,000 companies worldwide love Campaign Monitor.

From Australia to Zimbabwe, and everywhere in between, companies count on Campaign Monitor for email campaigns that boost the bottom line.

Get started for free