Latest / The Email Marketing Podcast with Fexingo: Newsletters, Open Rates, and Subscriber Growth / How One Line in a Subject Line Lifts Open Rates by 40 Percent
Transcript
- Lucas: Let me start with a number that stopped me this week: a single change to a subject line — one short phrase — lifted open rates by 43 percent in a B2B SaaS newsletter I've been following. Luna: Forty-three percent from one line? That sounds too clean. What was the change? Lucas: The company had been sending their weekly digest with a straightforward subject line — think 'Your Weekly Product Roundup' — and they swapped it to something that started with a teaser: 'The one metric you're ignoring' and then the actual topic. So the full subject line became: 'The one metric you're ignoring: Your Weekly Product Roundup.' Open rate went from 22 percent to 31.5 percent. Luna: So they added a curiosity gap before the colon. That's a classic copywriting move — but 43 percent lift is huge. Was this before or after Apple's Mail Privacy Protection rolled out? Lucas: After — this was late last year. And that's actually the interesting part. Since Apple's update, open rates are less reliable as a metric, but when you see a jump that big and you're also tracking click-through rates, you can still trust the directional signal. Their click-through rate went up 18 percent on those same emails. Luna: So the curiosity gap didn't just get people to open — it got them to read more. That's the real win. What was the exact phrase they added? Was it always 'The one metric' or did they test variations? Lucas: They tested three versions over four weeks. The control was their usual descriptive subject line. Version A added 'The one thing...' before the topic. Version B added 'You're missing...' And Version C combined both: 'The one thing you're missing:.' That last one won by a wide margin — 43 percent over control. Version A was 22 percent lift, Version B was 15 percent. Luna: So the combination of specificity — 'the one thing' — plus a slight negative framing — 'you're missing' — created a stronger pull than either alone. That's a useful takeaway. Lucas: Exactly. And this isn't a new idea — it's a version of the 'curiosity gap' that's been studied in psychology for decades. But what's interesting is how well it still works in an era where people's inboxes are flooded and Apple's privacy changes have made open rates less precise. The gap still drives behavior. Luna: Right. And I think there's a nuance here that's easy to miss: the curiosity gap only works if the email itself delivers on the promise. If you open an email promising 'the one thing you're missing' and it's just a generic update, you lose trust fast. Lucas: Absolutely. The SaaS company I mentioned — they made sure the email's first paragraph directly answered the teaser. The subject line said 'the one metric you're ignoring,' and the very first line of the email said: 'It's churn rate broken down by customer segment.' They delivered the punchline immediately. Luna: That's smart because it also respects the reader's time. They got the answer in the first sentence, so even if they didn't read the rest, they felt satisfied. That probably explains the click-through lift too — they earned enough trust to keep scrolling. Lucas: Let me walk through a second example, because the first one was B2B SaaS — a slightly different audience. A friend of mine runs a newsletter for freelance designers, about 40,000 subscribers. She tried a similar curiosity-gap approach but with a twist. Luna: What was the twist? Lucas: She used a question in the subject line, but not just any question — a question that implied she had insider knowledge. So instead of 'How to raise your rates,' she wrote: 'Why I stopped charging hourly.' Open rate went from 28 percent to 39 percent. Luna: That's a 39 percent lift — nearly identical to the SaaS example. And the subject line is more personal, more specific. It's not 'here's a tip,' it's 'here's what I actually did.' That's a different kind of curiosity: social proof plus a promise of a concrete number. Lucas: Exactly. And she also tested a version that included a number in the subject line: 'How I doubled my income in six months' — but that only lifted open rates by 11 percent. The version with the personal story and the specific behavioral change — 'stopped charging hourly' — did much better. Luna: So numbers alone aren't a magic bullet. It's the combination of a specific, unexpected action — 'stopped charging hourly' — that triggered curiosity. The number just confirmed it was worth reading. Lucas: Right. And this is where the data gets really useful for email marketers: you can A/B test subject lines with relatively small sample sizes and get statistically significant results in a few hours if your list is over 10,000. The freelance designer's test ran for just 6 hours before she had a clear winner. Luna: Six hours — that's fast. But you need enough volume. For smaller lists, you might need to run the test for a full send cycle, or use a tool that uses a holdout group. Lucas: Agreed. But the principle holds: curiosity gaps work, and they work especially well when the subject line is specific, personal, and promises a concrete answer. The more generic the curiosity — like 'You won't believe what happened next' — the worse it performs, because people have learned to distrust that phrasing. Luna: Yeah, that's the 'clickbait fatigue' factor. If it sounds like a tabloid headline, people tune it out. The curiosity has to feel earned and relevant to the audience. Lucas: Let's talk about the mechanics of testing this. If you're an email marketer listening and you want to try this tomorrow, what's the most reliable way to set up the test? Luna: I'd say: pick one variable — just the subject line — and use an A/B test with a 50-50 split. Send version A at your usual time, version B at the same time. Let the test run for at least 4 hours or until you have 1,000 opens per variant. Then check the open rate and click-through rate. Don't just look at opens — clicks tell you if the content delivered. Lucas: And what about the body of the email? Should you change anything else? Luna: No. Keep everything else identical. The subject line is the only variable. If you change the preview text or the first sentence, you can't isolate the effect. That's the most common mistake I see. Lucas: Good point. And one more thing: when you write the curiosity-gap subject line, make sure the gap is genuinely relevant to your audience. The SaaS company's audience cared about metrics, so 'the one metric you're ignoring' was perfect. The freelance designer's audience cared about pricing strategies, so 'why I stopped charging hourly' resonated. Luna: Exactly. If you're sending a newsletter about gardening tips, 'the one pest you're ignoring' might work. If you're sending a finance newsletter, 'the one tax deduction you're missing' could be powerful. It's about matching the curiosity to the audience's existing interests. Lucas: I want to bring in one more data point. A 2025 study from Litmus analyzed over 50 million emails and found that subject lines with what they called a 'specific curiosity trigger' — a phrase that hints at a concrete, surprising fact — outperformed generic subject lines by an average of 32 percent in open rate, across all industries. Luna: Thirty-two percent average — that's massive. But I'd love to see the breakdown by industry. I bet B2B tech and media newsletters see the biggest lift, while e-commerce sees a smaller one because their audience is more transactional. Lucas: The study did break it down. B2B software saw a 38 percent lift. Media and publishing saw 35 percent. E-commerce saw 22 percent. So you're right — transactional emails are a different beast. But even 22 percent is significant if you're sending millions of emails a month. Luna: That's worth its own episode — how to write subject lines for e-commerce versus newsletters. But for now, I think the takeaway is clear: one well-crafted curiosity line in your subject line can be the highest-leverage change you make this week. Lucas: Yeah. And it costs nothing but a few minutes of thoughtful writing. If today's episode was useful to you and you want to keep it ad-free, buy me a coffee dot com slash fexingo helps. That's literally what keeps these conversations going. Luna: Absolutely. Even a small contribution makes a difference. And it keeps us independent, which we value a lot. Lucas: So back to the subject line strategy — there's one more nuance I think is worth mentioning. The length of the curiosity phrase matters. The SaaS company's winning subject line was 58 characters. The freelance designer's was 44 characters. Both are on the longer side for subject lines, but they worked. Luna: That's counter to the old advice that shorter subject lines always win. It suggests that specificity and curiosity can overcome the length penalty, especially on mobile where you have less room. Lucas: Exactly. And mobile open rates are over 60 percent now for most B2C lists. So the challenge is: how do you fit a curiosity gap into the 40-50 characters that show on a phone screen? The SaaS company put the curiosity trigger right at the beginning — 'The one metric you're ignoring' — so that shows up even on a small screen. The colon and the rest might get cut off, but the hook is visible. Luna: That's a key tactical tip: front-load the curiosity. Don't bury it after a long introductory phrase. Put the most intriguing words first. Lucas: Right. And if you're using preheader text — the snippet that appears after the subject line in most email clients — use that to reinforce the curiosity rather than repeat it. For example, the subject line could say 'The one metric you're ignoring' and the preheader could say 'It's not what you think.' That extends the gap. Luna: That's a great technique. Preheader text is often underutilized. It's like a second subject line. You can use it to add another layer of curiosity or to qualify the promise. Lucas: One caution: don't make the preheader too mysterious or it might seem like clickbait. The preheader in that example — 'It's not what you think' — works because the subject line already set up a specific expectation. The preheader just adds a twist. Luna: Right. It's a small tweak, but it can have a big impact. I've seen tests where adding a preheader that teases a specific benefit increased open rates by another 10 to 15 percent on top of a curiosity-gap subject line. Lucas: So the full formula might be: a curiosity-gap subject line with the trigger front-loaded, plus a preheader that deepens the mystery or adds a benefit. Then the first paragraph of the email delivers the answer immediately. That's a winning structure. Luna: And then you A/B test it against your current control to make sure it works for your specific audience. Because no formula is universal — but this one has a strong track record across industries. Lucas: Let's do a quick recap for anyone taking notes. One: identify a specific, surprising fact or insight your audience cares about. Two: phrase it as a curiosity gap — 'The one you're ' or a personal story like 'Why I stopped.' Three: front-load the trigger in the subject line. Four: use preheader text to reinforce. Five: deliver the answer in the first sentence of the email. Six: test it against your current best performer. Luna: That's a solid checklist. And I'd add: don't overuse it. If every email uses the same curiosity structure, it loses its power. Save it for your most important sends. Lucas: Good point. Frequency dilutes impact. Use it for your best content — a deep dive, a new data set, a personal story — not for every weekly update. Luna: So what's next for you on this topic? Are you going to test this on your own newsletter? Lucas: I already started. I'm running a two-week test on my personal newsletter, which goes to about 8,000 people. I'll share the results in a future episode if they're interesting. The control is my usual descriptive subject line — think 'Issue #142: Marketing Trends' — and the test is a curiosity-gap version. Luna: I'd love to see the data. I might run a similar test on our show's weekly email update. The subject line is usually 'New Episode: ' — I could test 'The one email trick you haven't tried' or something similar. Lucas: That would be a great test. And because we have a relatively small but engaged list, we'll need to run it for a full send cycle to get significance. But I think it's worth it. Luna: Alright, I'm convinced. I'll set up the test this week. Thanks for the deep dive — this was one of those episodes where I walked away with something I can use immediately. Lucas: That's the goal. One concrete takeaway per episode. If today's was useful, share it with a colleague who's struggling with open rates. And we'll be back next time with another angle.