Latest / The Newsletter Podcast with Fexingo: Substack, Beehiiv, and Email Newsletter Business / How Newsletters Use Subject Line Personalization to Boost Open Rates
Transcript
- Lucas: We talk a lot about growing a newsletter audience, but there is one metric that determines whether your content even gets seen: the open rate. And the quickest way to move that number is a subject line that feels written just for that one person. Luna: Right, but we are past the era where putting the subscriber's first name in the subject line is enough. That ship sailed maybe five years ago. Lucas: Fully agree. The bar has moved to what I would call behavior-driven personalization. I want to focus on a specific case that makes this concrete. There is a mid-size B2B newsletter called The Lead — roughly fifty thousand subscribers — that ran a test earlier this year. Their standard subject line format was something like 'Your Weekly Marketing Roundup.' For one segment, they swapped in a subject line that referenced the last article the subscriber clicked on. Luna: So if I clicked a piece on LinkedIn ads, my subject line would mention LinkedIn ads? Lucas: Exactly. The personalized version read like 'Your next move on LinkedIn ads' — directly referencing the topic they had engaged with. Across a four-week A-B test, the personalization lifted open rates by forty-two percent compared to the generic control. That is not a marginal gain. Luna: Forty-two percent is huge. But the obvious question is: how do you set that up without a massive engineering lift? Most newsletter platforms have merge tags, but tracking individual click behavior and feeding it back into the subject line is a different beast. Lucas: It is, and this is where the platform choice matters. The Lead runs on Beehiiv, which has a custom field feature that lets you import data from your own analytics — or you can use their native click tracking to populate a field called 'last_clicked_topic.' Substack doesn't have that level of flexibility today; you would need a workaround like a third-party tool that syncs with the Substack API. But the principle is the same: you need a database that stores a subscriber's recent behavior and a template that pulls that into the subject line. Luna: And you need to decide what behavior qualifies. Is one click enough? Or do you need a pattern? Lucas: Good point. The Lead used a two-click minimum over a seven-day window before the field updated. They wanted to avoid a one-off accidental click triggering a subject line that felt random. The rule was: if someone clicked two articles on the same broad topic category within a week, the subject line would reference that topic. Otherwise, it defaulted to the generic roundup format. Luna: Smart. That reduces noise. But there is a psychological line you don't want to cross. If a subscriber feels like you are watching their every move, it can feel creepy rather than helpful. We saw that with some of the early retargeting ads. Lucas: Absolutely. The threshold for creepiness is lower in email because inbox is a private space. So the rule of thumb is: the personalization should feel like a helpful reminder, not a surveillance report. Mentioning a broad topic category works. Mentioning the exact article title or the time of day they opened — that starts to feel invasive. Stick to the category. Luna: And that is exactly the kind of insight that takes a newsletter from average to must-read. It's also the reason why we keep this show free from ads — we want to talk about what actually works, not what a sponsor wants us to say. Lucas: Yeah, that's a core part of our mission here. We deliberately don't run ads on these episodes. So if you find value in conversations like this and want to support that choice, the link is buy me a coffee dot com slash fexingo. Every contribution helps us stay independent and focused on what matters for newsletter operators. Luna: Exactly. Now, back to subject lines — let's talk about the data infrastructure. What did The Lead actually need to set up? Lucas: They needed three things. One: a way to tag every article with a topic category — in their case, they used about eight categories like 'paid ads,' 'SEO,' 'content strategy,' and so on. Two: a custom field in Beehiiv that stored the subscriber's most recent category. Three: a logic rule in their email template that said 'if custom_field is not empty, use personalized subject line; else use default.' The whole setup took their engineer about half a day. Luna: Half a day for a forty-two percent lift. That is an incredible return on time. Lucas: It really is. And the lift held across multiple weeks — it wasn't a novelty effect. In fact, the personalized group had a slightly lower unsubscribe rate over the test period, which suggests the content felt more relevant overall. Luna: That makes sense. If the subject line sets a relevant expectation, the reader is more likely to find the content useful, and they are less likely to tune out long-term. Lucas: Right. So the takeaway is not just 'personalize your subject lines.' It is 'personalize based on behavior, use broad categories, and set a minimum engagement threshold before triggering the personalization.' That last point is crucial because it also prevents the system from personalizing based on stale behavior. If someone clicked an article six months ago, you don't want to reference that today. Luna: So you need a recency window as well. The Lead used seven days. That seems tight — what about subscribers who only open once a month? Luna: They would get the generic subject line, which is fine. You don't want to force personalization where you don't have current data. A generic but well-written subject line outperforms a stale personalized one every time. Lucas: Exactly. And this brings up another nuance: the personalized subject line also needs to be tested for length. If you insert a long topic name like 'account-based marketing strategies,' it might get truncated on mobile. The Lead kept their category names short — two to three words max. Luna: That is a practical detail that is easy to overlook. So the categories need to be designed for mobile screens from the start. Lucas: Yes. And they also tested two variants of the personalized subject line: one that asked a question — 'Stuck on LinkedIn ads?' — versus one that made a statement — 'Your next move on LinkedIn ads.' The statement format won by about six percent in open rate. But I imagine that could vary by audience. Luna: Probably. A more analytical audience might respond better to a question that invites curiosity. But the key is that they tested it. A lot of newsletter operators would just pick one format and never revisit. Lucas: And that is exactly the mindset we want to encourage. Treat your subject line like a landing page headline — it deserves its own optimization process. The fifty thousand subscribers of The Lead represent a solid sample size, but even a list of five thousand can produce statistically significant results with a well-structured A-B test. Luna: So what about deliverability? Could personalization trigger spam filters? Some filters flag subject lines that look too tailored because they resemble phishing patterns. Lucas: It is a legitimate concern. But in practice, as long as you are using the subscriber's own behavior data and not inserting personal information like their name or email address into the subject line, most filters are fine. The Lead saw no change in inbox placement rate during the test. The key is to avoid anything that looks like a password reset or account notification — keep the language natural and benefit-oriented. Luna: So 'Your next move on LinkedIn ads' passes the sniff test. 'We noticed you clicked on LinkedIn ads' — that might feel too surveillance-like and trigger suspicion. Lucas: Exactly. Frame it as a service, not a report. The subscriber should feel like the newsletter is anticipating their interests, not tracking their every click. Luna: I think that is the golden rule for all personalization: it should feel like a thoughtful gift, not a shadow. Lucas: Well said. So if you are running a newsletter today, I would encourage you to look at your subject line performance over the last month. Pick the three worst-performing subject lines and ask yourself: could I have written a version that references a topic this person has shown interest in? If the answer is yes, and your platform supports it, run a two-week test. You might be surprised. Luna: And if your platform doesn't support it natively, there are middleware tools like Zapier or custom API integrations that can sync click data into a custom field. It is not trivial, but it is doable for a technical founder or a freelancer. Lucas: Right. The barrier is lower than most people think. And the upside — a forty-two percent open rate lift — is hard to ignore. That is the kind of improvement that can turn a struggling newsletter into a growing one. Luna: Alright, so next week we are going to look at the flip side: what happens after the open. We will talk about how to use personalization in the body of the newsletter to increase click-throughs and reduce unsubscribes. Lucas: Looking forward to that. For now, if this episode sparked an idea you want to try, go ahead and set up that custom field. And if you found it useful, you know where to find us — buy me a coffee dot com slash fexingo. Thanks for listening.