The report was based on data from more than 3 billion attempted email sends that were part of 125,000 global campaigns sent by brands in four industries (retail/B2C, conferences/events, media/publishing, and B2B). The researchers selected the 300 most popular single words, characters, and two-word phrases and then looked at the relative lift or decay of each term versus the average.
As the report notes, the results only provide a "broad interpretation of the causal effects of individual words"; that is, they are a general overview of average performance and the terms won’t always work for every brand.
Overall, email subject lines that include the words "thank you" have the highest above-average engagement levels (+62%)—perhaps because many automated, transactional messages include this phrase, such as email receipts sent by brands after customers complete online purchases.
Subject lines that separate topics with pipes (e.g., "Sale now on | New lines added | Win trip to Dubai | Share your pics with #summer2015rules") also perform significantly above average (+47%).
Timeliness also tends to work well in email subject lines, with words such as "bulletin" (+32%), breaking (+27%), and "order today" (+27%) all boosting engagement.
Not surprisingly, mundane words that make reading feel like work hurt engagement levels with email subject lines.
The worst-performing words examined include: "journal" (-50%), "forecast" (-47%), "training" (-47%), "whitepaper" (-40%), and "learn" (-36%).
Check out the full report for results by category, as well as for the specific engagement metrics (open rate, CTR, unsubscribe rate, etc.) of individual words.
About the research: The report was based on data from more than 3 billion attempted email sends that were part of 125,000 global campaigns sent by brands in four industries (retail/B2C, conferences/events, media/publishing, and B2B).