Email marketers have spent years following the same advice: use strong words, personalize subject lines, avoid ALL CAPS, and keep punctuation to a minimum. New research suggests that some of those laws may be hurting performance instead of helping it.
Researchers at the University of Helsinki analyzed the subject lines of 31,812 marketing emails sent 4.6 billion times. Their goal was to find out which coding conventions have been around for a long time when tested at scale. The results challenge several conventional “best practices” and emphasize the importance of testing assumptions rather than relying on conventional wisdom.
The biggest surprise of the study may have been its discovery of so-called powerful words.
Terms like for free, exclusive, today, flashagain save appeared in email marketing tips for years. They are often presented as reliable ways to increase openings. In research, subject lines containing these words generated significantly lower open rates than those without.
It is possible that these words are so familiar that they no longer stand out. In crowded inboxes, specific language may gain more attention than predictable sales phrases.
ALL CAPS also worked against advertisers. Subject lines containing capitalized words decreased open rates by about 3.3%, suggesting that subscribers continued to respond negatively to what sounded like shouting.
The study also confirmed that shorter courses were consistently more effective. Each additional letter had a small negative impact on open rates, reinforcing the value of concise writing.
Some rules are worth checking out
Some findings were less clear, but still useful.
Personalization produced modest improvements in open standards. The researchers also noted that previous studies reached conflicting conclusions depending on the audience and stage of the shopping journey. Their recommendation is to test whether personalization improves results for your subscribers rather than assuming that it will.
Punctuation also produced an unexpected result. Subject lines containing a single exclamation point increased open rates by nearly 4%. The findings support using less emphasis while avoiding excessive punctuation commonly associated with spam.
The researchers also found that a slight departure from traditional writing, including creative punctuation or formatting, was associated with higher open rates. Carefully breaking the rules can help an email stand out from a crowded inbox, as long as it’s legible and professional.
What should be checked by sellers
The findings also provide useful guidance for teams using AI to write email campaigns.
Many AI writing tools generate subject lines based on these copy best practices. As a result, asking AI to generate “highly dynamic” subject lines full of urgency and promotional language can automatically create outdated practices.
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Better to use AI to generate diversity, then examine the thinking behind it. Compare a straightforward subject line versus one full of power words. Check the customization against the standard version. Try to copy short from something long. The goal is to learn what writing principles work for your audience.
Marketing history is not proof
The big lesson goes beyond email.
Marketing has accumulated decades of rules, formulas, and best practices that are repeated in conference sessions, agency presentations, and online guides. Some survive because they always work. Some survive because they are repeated often enough to become accepted wisdom.
Marketers who treat established advice as a hypothesis, test it with their audience, and measure results have a solid foundation for deciding which practices to keep in their playbook.
The interestingly named study, “Do you want $150 FREE? Estimating the effect of language on open rates of email marketing,” was published in the journal Ampsand, and can be downloaded here.
