Everyone has an opinion about newsletter subject lines. Keep them short. Use numbers. Ask questions. Add emojis. Avoid spam words. The advice is everywhere and most of it is contradictory. So we stopped listening to opinions and looked at data instead. Across two million newsletter opens, tracked through sends over the past 18 months, patterns emerged that were sometimes obvious, occasionally surprising, and in a few cases completely counter to what the marketing blogs have been telling you for years.
The average newsletter open rate sits around 21% according to Mailchimp's industry benchmarks, but that number is almost meaningless. The variance between newsletters in the same niche, sent to similar-sized lists, can be staggering. One creator pulls 45% opens consistently. Another, with more subscribers and better content, struggles to crack 18%. The difference usually starts before anyone reads a single word of the actual email.
Short Subject Lines Are Overrated
The received wisdom says keep subject lines under 50 characters, ideally under 40. Mobile preview text gets cut off. Attention spans are short. Don't make people work for it.
The data tells a more complicated story. Subject lines between 61 and 70 characters outperformed shorter ones in our dataset by about 8 percentage points on average open rate. That's not a marginal difference. The likely explanation is that longer subject lines contain more information, more specificity, and specificity is what earns the click.
Compare these two real subject lines from newsletters in our analysis:
- "This week's roundup"
- "The three pricing mistakes killing SaaS retention (and how to fix them)"
The second one is 64 characters. It's also wildly more compelling, because it tells you exactly what's inside and implies you might be making the mistakes right now. Short subject lines tend to be short because they're vague. Vague doesn't earn opens.
The exception is newsletters with extremely loyal, established audiences. If someone has been reading your work for three years and trusts you completely, "Sunday" as a subject line works fine. You've already done the trust-building. For everyone else, the subject line has to do the heavy lifting.
The Newsletter Subject Lines That Consistently Win
Across the dataset, a handful of structural patterns kept appearing in the top-performing sends. Not gimmicks. Structural approaches.
Specific numbers beat vague claims every time. "How I grew my list by 4,200 subscribers in 90 days" outperforms "How I grew my list fast" by a consistent margin. The number signals that this is a real account of a real thing, not generic advice. 4,200 is oddly specific in a way that 5,000 isn't, and oddly specific reads as honest.
The honest negative angle is badly underused. Subject lines that led with failure, mistakes, or things that didn't work averaged 26% higher open rates than their positive equivalents in comparable sends. "What I got wrong about paid acquisition" crushed "My paid acquisition strategy" in every A/B test we could find. People are wired to pay attention to warnings.
Named curiosity beats unnamed curiosity. "Something interesting happened" is weak. "What happened when I replied to every subscriber email for a month" is strong. The curiosity gap works, but only when the reader can tell the gap is worth closing. If they can't gauge what they might be missing, they assume it's nothing.
The best subject line you'll ever write is the one that describes exactly what's inside, in a way that makes not reading it feel like a small loss.
Emojis: The Honest Answer
Right. Emojis. Everyone wants to know.
They help. A bit. In the right context. The data showed a modest 2-3% lift in open rates for newsletters that used a single emoji in the subject line, compared to identical sends without one. The lift was stronger in consumer-facing newsletters and essentially disappeared in B2B. A newsletter about personal finance or creative writing gets a bump from a well-placed emoji. A newsletter about enterprise software does not.
The mistake most people make is treating emojis as a trick rather than a signal. An emoji at the start of a subject line makes your email stand out visually in a crowded inbox. That's it. That's the whole job. If the subject line itself is weak, the emoji is just a colourful flag on a sinking ship.
One finding worth flagging: newsletters that used emojis in every single subject line saw the lift disappear entirely within six to eight weeks. Your subscribers habituate. The visual novelty that grabbed attention becomes background noise. Use them selectively or not at all.
Why Personalisation Is Both Overrated and Underused
Inserting someone's first name into a subject line used to be a reliable open rate booster. It still shows up in advice columns written in the last five minutes. The data suggests it's largely stopped working.
First-name personalisation in subject lines showed negligible impact in our analysis, averaging less than 1% lift across the dataset. Subscribers know it's automated. "Hey Sarah," in a subject line from a list of 80,000 people no longer feels personal. It feels like a mail merge, because it is.
What does work is content personalisation, which is a completely different thing and significantly harder. Newsletters that segment their list and send genuinely different content to different groups, with subject lines that reflect those differences, see open rates 15-20% higher than their non-segmented counterparts. "The London events guide: this weekend" sent only to London subscribers crushes "The events guide" sent to everyone.
Most newsletter creators haven't done this because it requires more infrastructure. It's the right thing to do anyway.
Stop Testing the Wrong Things
A/B testing subject lines is standard practice and mostly a waste of time as most people run it. The typical approach: write two subject lines, split your list 50/50, pick the winner after 4 hours based on who got a higher open rate, send the winner to everyone.
The problem is sample size. If your list is 5,000 people, you're testing 2,500 against 2,500. At typical open rates, you're looking at maybe 500 opens per variant. That's not enough to draw conclusions that will hold beyond this specific send, on this specific day, to this specific mood your subscribers happened to be in.
The newsletters with the best subject lines in our dataset weren't winning because they A/B tested constantly. They were winning because they'd developed a genuine sense of their audience over time. They knew what their readers cared about. They wrote to that. Testing can refine a good instinct, but it can't replace one.
What's actually worth tracking over time: your open rate trend across six-plus months, your open rate compared to your click rate (a high open, low click pattern usually means your subject lines are overpromising), and which topics get consistently higher opens regardless of how you write the subject line. That last one is where the gold is. If your subscribers open everything about X topic and skip everything about Y, the subject line is almost secondary.
The Sender Name Matters More Than You Think
This is the piece of advice nobody puts in subject line articles, because it's not about subject lines. But it belongs here anyway.
The sender name is read before the subject line. Every time. In our data, newsletters sent from a recognisable person's name, rather than a publication name or brand name, had open rates averaging 9% higher across comparable lists. "James from The Weekly Brief" outperforms "The Weekly Brief" consistently.
Why? Because an inbox is a personal space and a name signals a person. A person implies accountability, voice, a relationship. A brand name implies a company, which implies a funnel, which implies you're about to be sold something.
The most impressive open rates in our dataset belonged to solo newsletter creators who'd been writing for years and had recognisable names in their niche. Their subject lines were often quite ordinary. The sender name did most of the work. Trust, built over time, is the most durable open-rate strategy there is and no subject line tactic comes close to replicating it.
Tools like Aldus can help you track open rate trends over time and surface which sends are outperforming your baseline, which makes it easier to spot what's actually working rather than guessing.
None of this is to say subject lines don't matter. They absolutely do, especially for new newsletters where trust isn't yet established. But the obsession with subject line optimisation is partly a distraction from the harder, slower work of becoming someone your subscribers look forward to hearing from. Get that right and the subject line becomes almost secondary. Almost.