Latest / The Email Marketing Podcast with Fexingo: Newsletters, Open Rates, and Subscriber Growth / How Email Subject Lines Affect Subscriber Trust
Transcript
- Lucas: So there's this data point that's been stuck in my head since I read the 2025 Litmus and HubSpot email benchmarking study. Lists that optimized subject lines purely for open rate lost about 23 percent more subscribers over a 90-day window than lists that optimized for what they called 'trust-first' copy. Luna: Twenty-three percent — that's not just a blip. And trust-first in this context means what, exactly? No fake urgency, no misleading teasers? Lucas: Exactly. Specific, honest, and consistent with the brand's voice in other channels. The study tracked six months of email campaigns from about 800 mid-market brands. The ones that avoided phrases like 'RE:' in the subject line when there was no prior conversation, or 'Your account is at risk' when nothing was actually wrong — those lists showed significantly lower spam-complaint rates and higher reply rates. Luna: Right, because reply rate is a signal that someone actually wanted to engage, not just open. Lucas: Yeah. And that's the shift I want to talk about today. For years, the subject line conversation has been entirely about getting the open. But if you optimize for open rate alone, you're optimizing for a metric that can be gamed — and gaming it erodes the asset you're actually trying to build, which is a list of people who trust you. Luna: Let's get concrete. Can you give an example of a brand that did this well and one that got burned? Lucas: Sure. On the burned side, there's a DTC pet-supply brand that I won't name — they ran a 'Your dog's health alert' subject line campaign to push a flea treatment. Open rates spiked to 48 percent, but within two weeks, their unsubscribe rate tripled and they got flagged by Gmail's spam filter. They recovered, but it took months. Luna: Ouch. And on the trust-first side? Lucas: A good example is a brand like Riveter — the women's co-working space that scaled into a newsletter. They use subject lines that are almost boringly descriptive: 'Your March events at Riveter Union Market' or 'New Riveter locations opening this fall.' Nothing clever. Their open rates are in the low 30s, which is fine, but their list to engagement deviation — I'll explain that term in a second — is basically zero. People who open tend to click, and people who click tend to stay. Luna: So they're trading peak open rate for consistent engagement over time. That's a list-health play. Lucas: Exactly. And I want to give you three signals you can look at in your own ESP to diagnose whether your subject lines are becoming toxic before the churn data catches up. The first is what I call the reply to spam ratio. Luna: I like that. Let me guess — if you're getting more spam complaints than replies, your subject lines are probably overpromising. Lucas: Bingo. Most ESPs give you a complaints rate and a reply rate. If your reply rate is below 0.1 percent and your complaint rate is above 0.05 percent, you're in the danger zone. The second signal is list to engagement deviation. Luna: Walk me through that one. Lucas: This is a metric I adapted from a presentation by an email consultant named Jason Rodriguez. You compare your overall list open rate to the open rate of your most engaged segment — say, people who opened in the last 30 days. If the gap between them is wider than 20 percentage points, it means you're getting a lot of opens from low-intent subscribers who are being tricked by the subject line. And those are the people who will eventually mark you as spam. Luna: That's a smart diagnostic. Because if your best subscribers are opening at 45 percent and your overall list is at 25, that's a healthy gap driven by relevance. But if your overall list is at 40 and your best segment is at 45, something is pulling in disengaged people. Lucas: Exactly. The third signal is the bait score. This is something you can calculate manually: take the percentage of subject lines in your last 20 campaigns that contain urgency words — 'limited time,' 'ending soon,' 'last chance,' 'RE:' — or question marks that imply a benefit not delivered in the body. Then compare that to your unsubscribe rate for those same campaigns. If the bait score is above 40 percent and your unsubscribe rate is above 0.5 percent, you have a subject-line toxicity problem. Luna: That's simple enough to run in a spreadsheet. I'm going to do that for a client this week, actually. They've been leaning hard on 'Your cart is expiring' subject lines even for people who haven't added anything to cart. Lucas: Yeah, that's a classic. And the thing is, it works in the short term. That's why people keep doing it. But the cumulative effect on list health is real. I want to zoom out for a second — because this isn't just about subject lines. It's about the broader principle that every email you send is a transaction of trust. The subject line is the first impression, but it sets an expectation for the content. If the content doesn't deliver, the subscriber learns not to trust you. Luna: And trust is really hard to rebuild once it's broken. You can always get a new open, but you can't always get that subscriber back. Lucas: Right. And this is exactly why conversations like this one matter. You know, if these marketing conversations have sparked something you've actually used — a tactic, a framework, a way of looking at your list — a couple of dollars a month is genuinely what keeps these going. Buy me a coffee dot com slash fexingo, if you've gotten something out of them. It makes a real difference. Luna: Yeah, it does. It's what keeps us ad-free and focused on the stuff that actually helps. Lucas: So back to the bait score — once you've identified that you have a problem, the fix isn't to write boring subject lines. It's to write honest ones that still create curiosity. There's a framework from copywriter Joanna Wiebe that I like: the subject line should make a promise that the email body specifically fulfills. If the email is about three ways to save on shipping, the subject line should say '3 ways to save on shipping' or 'Your shipping costs, slashed' — not 'You won't believe what we did to shipping.' Luna: Because the second one sets up a surprise that the email can't deliver on. It's a mismatch. Lucas: Exactly. And the data shows that mismatch leads to higher complaint rates over time. The Litmus study tracked a cohort of subscribers over six months and found that those who received a 'surprise and delight' subject line — something vague like 'What we did for you' — were 1.7 times more likely to unsubscribe by month six than those who got a specific subject line like 'Your free shipping upgrade is ready.' Luna: So specificity builds trust. That makes sense. But what about personalization? There's been a trend of putting the subscriber's first name in the subject line, but I've read that it can backfire if overused. Lucas: Yeah, personalization is a double-edged sword. The same study found that first-name subject lines boosted open rates by about 11 percent on average, but they also increased spam complaints by 8 percent when used in more than 30 percent of a brand's campaigns. The theory is that if you use it every time, it becomes noise — and for some recipients, it feels like a sales tactic rather than a personal touch. Luna: So frequency matters. Use it sparingly, and only when it's actually relevant to the content. Lucas: Right. And the content should justify the personalization. If the email is about a product recommendation based on past purchases, using the name makes sense. If it's a newsletter roundup, probably not. Luna: Okay, so let's say I'm a marketer listening to this. I've run the bait score, I've checked my reply to spam ratio, and I find I have a problem. What's the first thing I do to fix it? Lucas: Start with a 30-day detox. For the next 30 days, every subject line has to pass two tests: one, it must be a literal description of the email content. Two, it must not contain any urgency or scarcity language unless the offer genuinely expires within 48 hours. You'll probably see a temporary dip in open rates, but your engagement quality should improve. And after 30 days, run the diagnostics again. I'd bet your reply to spam ratio improves. Luna: That's a concrete action. And it's reversible — if it doesn't work, you can always go back to old tactics. But I suspect most people won't want to. Lucas: I agree. Because once you see the relationship between subject line honesty and list retention, it's hard to unsee it. And retention is where the long-term revenue lives.