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The Most (and Least) Effective Keywords in Email Subject Lines

by Ayaz Nanji  |  
July 22, 2013

Emails with the word "alert" in their subject lines have a 38.1% higher than average open rate and 61.8% higher click rate, according to a recent study by British marketing firm Adestra.

The keywords "free delivery" (+50.7% higher open rate, +135.4% click rate) and "bulletin" (+15.8%, +12.7%) also performed very well in the email campaigns analyzed.

On the other hand, "report" (-23.7% average lower open rate, -54.8% click rate), "learn" (-35.5%, -60.8%), and "book" (-4.6%, -25.4%) had a negative effect. "Newsletter" showed a marginal effect on open rates (+0.7%), but had an adverse effect on click rates (-18.7%.)

As for date-related keywords, "daily" (+27.8%, +100.3%) and "weekly" (+27.1%, +50.6%) performed strongly, but "monthly" (-26.6%, -37.0%) had a negative effect.

Below, additional email subject line keyword performance broken out by B2B, B2C and commerce. For complete results and analysis check out the full study, The 2013 Adestra Subject Line Analysis Report, which was based on a review of over 2 billion global emails.

B2B Emails

  • The words "alert" and "breaking" in the email subject lines of B2B emails performed well.
  • B2B customers seem to have become desensitized to words such as "reports," "forecasts," and "intelligence."

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Ayaz Nanji is an independent digital strategist and a co-founder of ICW Content, a marketing agency specializing in content creation for brands and businesses. He is also a research writer for MarketingProfs. He has worked for Google/YouTube, the Travel Channel, AOL, and the New York Times.

LinkedIn: Ayaz Nanji

Twitter: @ayaznanji

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  • by Aslam B Mon Jul 22, 2013 via web

    Great article and offers some very helpful tips! Thanks for sharing.

  • by Fran Simon Mon Jul 22, 2013 via web

    Effectiveness seems somewhat relative in this data. Oddly, some of the words with the highest open rates also had the highest rate of attrition (unsubscribes). It's almost as if the only reason the recipient opened the email was to unsubscribe. (I know that;s often the case for me!) I am also struck by how different the keywords are for B2B versus B2C. This is very intriguing data, but misleading if you think that Nirvana = open rate, or even CTR.

  • by Applicable to U.S. Market? Mon Jul 22, 2013 via web

    Adestra is a British company and I see that they tested with the symbol for the British pound. Were the email campaigns only in Europe and would the results even apply to the U.S.? I did download the report and in the amount of time I have to skim it I don't see information on the geography of the emails or the targets of the 90,000 emails.

  • by sj Mon Jul 22, 2013 via web

    Interesting article. As someone pointed out it might be more Britian specific but there's definitely enough of insight here for the globally saturating email market.
    Could someone please help me by decoding what *|*|*2, *|*|*3 & *,*,*4 etc imply? Thanks.

  • by Ayaz Nanji Mon Jul 22, 2013 via web

    The study doesn't break-down the email campaigns by geography, but I think it's safe to assume a very strong British skew. I'll keep a lookout for some purely U.S.-focused research.

    sj -- here's how the report explains the *|*|*: "This refers to a subject line that has multiple stories delineated by pipes. For example, Headline 1 | Headline 2 | Headline 3". Same thing for the commas ("Headline 1", "Headline 2", "Headline 3").

  • by sj Mon Jul 22, 2013 via web

    ah!! :) Thank You. (feeling foolish...but enlightened now)

  • by Ayaz Nanji Mon Jul 22, 2013 via web

    sj, my bad for not including that info in the first place!

  • by Kelly Mon Jul 22, 2013 via web

    My beef with this is that there is no mention whatsoever of spam filters. The word "spam" isn't even mentioned in the full report. Many of these words are those we've been told for years NOT to use in a subject line because of spam filters. Following Fran Simon's logic, it seems that some at least sound spammy to the recipient (ie: free delivery) because of the huge unsubscribe rate for those words, overall.

    And, what does it mean when the word "Subscription" in the subject gets only 5% increase in open rates overall and negative results in the three categories shown and ALL have very bad unsubscribes? Wouldn't the recipient have most likely *just* subscribed and are now receiving the double opt-in message? I know that's not always gonna be the case, but it just seems scary that that many people are leaving us stranded right after expressing enough interest to want to subscribe.

  • by Parry Malm Mon Jul 22, 2013 via web

    Hi all - I'm the author of the report. To clarify, the sample is roughly 40% UK and 60% ROW. So quite a bit of it is British, but quite a bit of it is not! It looks at long run correlations and not causations so don't bet your house on a magical subject line - use this as a thought generator but not the silver bullet :)

    And indeed, there's not much mention of spam... because the subject line has a very low effect on modern spam filter scoring. The majority is based upon your sender reputation, sending infrastructure and overall data quality (ie hard bounce rates etc.). Content of emails still matters, but much less than what it did back in the day.

  • by Applicable to U.S. Market? Mon Jul 22, 2013 via web

    Thanks for the clarification Parry!

  • by Ayaz Nanji Mon Jul 22, 2013 via web

    Thanks much Parry for jumping into the discussion.

  • by Pete_W Mon Jul 22, 2013 via web

    Interesting, but like other warnings here: don't be too myopic. Segmentation, relevant messaging and content (including timing), and ensuring the proper architecture and practices in your bigger e-mailing program will all (likely) have greater overall efficacy-impact than subject lines. Even those with seemily big % differentials-

  • by @AndreaMoe Wed Jul 24, 2013 via web

    This is great content and I see the question about applicability to the US and the comment from the author, "Hi all - I'm the author of the report. To clarify, the sample is roughly 40% UK and 60% ROW." I also emailed Adestra and they told me, "The data was 60% UK, 40% Rest-of-World, all English-language emails."

    It's still useful but as a B2B US only marketer I'll take the content with a grain of salt.

  • by Parry Malm Wed Jul 24, 2013 via web

    Hi Andrea - sorry, I made a typo. Indeed, roughly 60% UK it is. Grain of salt or not, even 1% of 2.2 billion is still a pretty big number... but ignore the report if you want, I won't take it personally :)

  • by @AndreaMoe Wed Jul 24, 2013 via web

    Not ignoring it at all! Thanks for the clarification and I really do appreciate the report. No really, I do :)

  • by Jas, Wed Jul 24, 2013 via web

    A great analysis; thanks to Parry Malm for clarifying the issue of correlation versus causation. Though generalisation of the data remains an issue, inclusion of a 40% ROW portion does give the study (with sample size, as I understand, of 5000+) some credit. As with any study, this should be taken as a great baseline for further exploration and analysis. Of course, marketing is a vast field, and as they say, individual results may vary. I would imagine that to successfully utilise this data, a business would "play around" with some of these keywords and measure their success using a combination of analytics and outcomes-based tools. Thanks for sharing, Ayaz Nanji.

  • by Iain Thu Jul 25, 2013 via web

    This is a great article and very insightful. I will put this to good use when formulating our subject lines.

  • by Gracious Store Thu Jul 25, 2013 via web

    It is not surprising that emails with "free shipping" in the subject lines have such high open and click through rates because people want free shipping and will definitely jump at it at the least opportunity

  • by Spook SEO Sat Aug 3, 2013 via web

    Thanks for sharing the data! I got tons of insight especially when looking at the numbers. Its amazing how a lot of the words with high open rates also have a high unsubscribe rate. Great post!

  • by dawn littlefield Fri Aug 9, 2013 via mobile

    It's interesting how call to action words that require the reader to "work" such as learn, read, subscribe, etc. Scored relatively poorly. Not surprising but a good reminder.
    Thanks for the article.

  • by Sandy Tue Aug 20, 2013 via web

    Can you clarify the difference between B2C and Commerce? I'm confused.

  • by Darren Leach Thu Aug 22, 2013 via web

    Articles like this never get old for me! Love learning about new ways in which I can improve my outreach!

  • by Lani Kirwan Tue Oct 8, 2013 via web

    Great summary of the study, data, e.g.. The charts make it much more clear, as to what figures you are dealing with. I agree with Parry, that this is not a magic bullet. This article does provide guidance, especially for newer marketers. What i thought was lacking, was that you did not address spam filters, at all. The article fails to address the fact that these statistics and data are from emails that make it into someones inbox. So, if you are not even making it through the spam filters, you need to address that issue first. Then you can start looking at your subject lines.

  • by Stacia Rubinovich Mon Oct 21, 2013 via web

    I'd be interested in seeing what impact some of these words had on delivery. Our research suggests that words like "free" tend to land emails in junk folders.

  • by Fluent Illustrator Thu Jul 23, 2015 via web

    Cant argue with actual numbers. Good read...

  • by Sophie Wilson Tue Oct 24, 2017 via web

    Amazing comparison! Could not think of so many email subject titles for A/B testing. Would be also very interesting to see how those titles perform compared to text+emoji titles. I'm running to set up my a/b testing campaigns in MailerLite.

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