"Does including 'free' in your subject line entice potential readers to open your message?"
In research they've done on subject line comparison, MailChimp found that using "free" has little impact on open rates in the media and publishing industry. "Interestingly enough," they write, "use of the word 'freebie' was found to result in a much larger increase in open rates." For me, the word "complimentary" feels more inviting than free. I just received an email with a subject line that reads "Complimentary ticket offer for 'Snow Child.'" The word "free" there would have made me suspicious. Complimentary has more heft. (The tickets went fast.)
Here are some other takeaways from their research:
Personalization works. "Our analysis found that personalization does indeed increase open rates. One of the most interesting findings is that, though the use of both first and last names in a subject is less common, it has the largest positive impact on open rates. Many of these campaigns seemed to contain highly personalized content." You don't see first and last names used as much—sounds worth testing.
Use urgency...sometimes. Words like urgent, breaking (as in breaking news) and important resulted in open rates that were much higher than normal. Just be careful not to overuse those words so they keep their impact.
Directness over trendiness. "Sometimes, it's better to be direct and descriptive than trendy," they write. "Seasonal slogans such as 'Fall into savings' or 'Sizzling summer bargains' are popular, but don't offer a specific hook. Instead, try to communicate the benefits of your promotions, or call attention to specific deals." I've also talked about using company anniversaries and milestones which seem more specific and celebratory than the seasons.
Word pairs that work. "Sometimes two words can provide context that a single word can't convey," MailChimp wrote. "First and foremost, people love to be thanked [so 'thank you' fares well]. It's also apparent that campaigns about current events, like natural disasters and political issues, have higher open rates than normal. And finally, it seems as though recipients don't like to be asked to sign up for anything—and they really don't like being told they're missing their last chance to get something they've already been emailed about." So while "sneak peek" did well, "sign up" and "last chance" performed terribly.
Long or short can work. "For most users, there is no statistical link between subject line length and open rate. But for subscribers reading your campaigns on mobile devices, shorter may be better." Test.
Announce and invite but don't hammer. "Recipients are much more intrigued by announcements and event invitations than cancellations and reminders. [We like being invited places.] It would appear that repeated reminders and cancellations don't pique their interest quite as much." I get lots of reminders for tickets that I have, and it's true, I quickly push them aside. More effective would be a subject line like, "Attending [this event]? Consider going to [this] or looking at [this complementary product]." Notice I used "going to" instead of "signing up for." (I'm a fast learner.)
Permission to capitalize. "...the use of an entirely capitalized subject line resulted in slightly higher open rates than usual for a given user/list."
Find the pain points. "I want you to forget all the channels and products that you produce and go back to, 'What are the pain points for your customers?'" Melcrum co-founder Victoria Mellor told us at BIMS. Use those pain points in your subject lines. "How to survive your next overnight flight." "Get more kitchen space with these easy fixes." "The best cities for finding a job."
Use vanity. To do this, you can either promise something that makes the subscriber look better to their peers or invoke the fear of being shamed." One example: "Gift inspiration for the discerning cyclist." The use of "discerning" there—or any positive adjective—works well as a subtle compliment. I'm discerning; I'll read this.
The more you test each type of subject line, the more you'll come to know your subscribers and their preferences.