Email Subject Lines That Work: How To Write & Optimize Yours

Why your email subject line matters

Since the iOS 15 update, the open rate craze has calmed down a little. Thanks to email-related privacy changes, this metric became far less reliable than it used to. You can’t assume that the percentages you see in your email marketing software are 100% accurate, and you can’t fully trust A/B tests targeting open rates as well. 

Here’s why subject lines still matter:

• Readers open emails less often. According to the report by Emfluence, average email open rates have been declining since 2019, with 2024 hitting the record low. So, to combat this tendency, you have to work towards making your emails attractive enough to open. Improving your subject lines is one way to do this.
• Subject lines actually affect email metrics. The recent extensive study by GetResponse showed that certain subject line characteristics don’t just affect open rates but also other, if not all, email metrics. For example, subject lines with the word “affordable” have the highest CTR of 11%, and subjects with the word “invitation” yield the most opens — a whopping 57%!
• Subscribers actually pay attention to them. A catchy subject line is not the only reason to open an email — your subscribers are likely to also check out the preheader and the sender info. However, it’s one of the elements they look at and one of the factors affecting the decision: open, ignore, or mark as spam? And, statistically, more opens means potentially more clicks and a higher chance of converting some of your subscribers into customers. 

Long story short, open rates are inflated and somewhat unreliable, but it doesn’t mean you shouldn’t work on improving the metric, and subject lines don’t affect just opens but clicks and even conversions.

The science behind effective subject lines

Firstly, we’ll have to disappoint you: there’s no foolproof formula for a subject line. Some studies contradict other studies, we don’t know your audience or email marketing struggles, and a magic word that will give you all the opens and clicks you can carry does not exist.

However, we found a couple of interesting stats and studies that might give you an insight into how to write subject lines.

❌ Out: Personalization

Many sources have suggested adding the email recipients’ first names in the subject lines to draw attention in the crowded inbox. Turns out, though, it doesn’t work quite as expected.

For the second year in a row, the GetResponse Benchmarks report shows that personalized subject lines actually perform worse. It’s not just about open rates — clicks also decrease by a whopping half.

The study by Attentive also shows that non-personalized subject lines for marketing emails outperform the personalized ones. However, it’s not true for triggered emails: since these are triggered by certain actions, the inclusion of personalization tokens feels more reasonable. 

The takeaway is, don’t focus on personalizing your subject line alone — personalize your email body instead, via variables and dynamic content. Simply calling your subscribers by their names is not enough to make them feel seen and valued!

❌ Out: Using spam words

Modern spam filters are far more complex than in the Y2K era of the internet, and they don’t just scan for specific email spam words. For example, Google has a bunch of AI-powered filters that keep learning and adapting to new phishing and scamming methods. 

However, here’s why you should consider avoiding spam words in subject lines:

• They might affect your reputation. If your subject lines consistently over-sensationalize minor events, use clickbait tactics and ALL CAPS, and induce anxiety with constant “last chances” and “limited offers,” you’ll be seen as spam — not by filters, but by your subscribers. And the ZeroBounce survey showed that 80% of people mark emails as spam because “they look like spam.” Sounds reasonable — other people won’t see you as credible if you’re writing emails like a Nigerian prince.
• They still might cause deliverability issues. If your emails have more than one potential risk factor, don’t add spammy subjects — otherwise, you’ll definitely end up in spam. For example, if you created a fancy but image-only email design, sending it with a subject like “AN IRRESISTIBLE OFFER EXPIRES NOW” will increase the risk of ending up in spam. Meanwhile, if you send the same email but with a normal subject line, you’re still at risk, but it will be lower.

❓ Maybe: Emojis

We’ve talked a lot about emojis in subject lines as a way to stand out visually and possibly increase your open rate. However, the numbers don’t go in line with that logic — according to the aforementioned GetResponse report, emojis drastically decrease open rates and increase unsubscribe rates too.

The Selzy team did our own emoji research and found the same patterns. However, when we investigated the data deeper, we found out that the lower metrics were actually attributed to poor-quality contact lists! We also found out that 32.4% of email campaigns sent via Selzy had 2 or more emojis in subject lines — that’s too much!

Should you use emojis in email subjects, then? Try and consider the following:

• Don’t overdo it — one emoji per subject is more than enough.
• Take email accessibility into account — that means a huge no to replacing words and letters with emojis, and refraining from using recently introduced ones since some of your subscribers may use old Android phones.
• Try non-conventional placements — for example, this study by SEJ showed that open rates increase with emojis at the end of subject lines.
• Consider generational and cultural differences — for example, Gen Z folks use the skull emoji (💀) in the same way Millennials and older folks use 🤣. Also, according to the Slack study, 🍑 is not always NSFW — for many Koreans, it’s literally a peach.

✅ In: Preheaders

According to Validity, brand recognition is the most important factor that makes or breaks your open rates. So, to improve your opens, the company suggests focusing on the combined power of your sender name, subject lines, and email preheaders too. Even email avatars can be impactful!

For example, this email’s subject line is “Illuminate the everyday.” The subject alone is good and intriguing, but it doesn’t tell us a lot. However, the preheader says “The Byredo Candle Edit.” That gives us a hint that we’re supposed to “illuminate” our lives with scented candles from a well-renowned niche perfume brand. 

Here’s why adding a preheader as a companion piece to your subject line makes sense:

• It helps you expand the subject line. It’s especially vital for mobile email optimization. You don’t have a lot of room when you only have the subject line. In this situation, a preheader allows you to add more relevant details and give a more convincing incentive to open the email.
• It looks more professional. If you don’t include anything specific in a preheader, the beginning of the email will show up automatically. And, if the first thing your subscribers see is something like “View the web version of the email,” it won’t make a good first impression.
• It can help you stand out. You can create an “empty preheader”, which is adding a bunch of spaces where your preheader should be — it will look like the email has no preview text at all. In some cases, it can work as an attention-grabber. Try and see for yourself!

✅ In: Keywords and numbers

Here are the most high-performing words, according to a study by MailerLite:

The results are obviously attributed to certain email types that naturally get opened more often, like welcome emails, emails with lead magnets, confirmation emails, event reminders and invitations, and so on. 

However, you shouldn’t dismiss the results of this study completely. Some of the higher-performing words can be used in a wider range of contexts. That gives you wiggle room for choosing more clickable synonyms. For example, say “Final hours” instead of “Last chance” in a subject line for a reminder email about a flash sale.

By the way, here’s the list of poor-performing words from the same study.

Some of these are clearly spammy, like “easy” and “missed,” or refer to certain verticals like “loan” and “game” that may refer to payday loans or gambling. You can still use this list to your advantage, though. For example, avoid the word “rate” while crafting subject lines for review request emails.

What about numbers then? Using numbers sets clear expectations — you’ll be seen as professional and candid, and such emails will be more enticing to open. Also, a clear definition of an offer in the subject line allows your subscribers to decide if they want it, instead of opening an email out of curiosity. That, in its turn, might indirectly affect not just opens but also clicks. 

Here are some examples we found on Milled:

How to write a perfect email subject line

While there’s no exact formula for the email subject line that will lead to a 100% open rate, there are some ways to increase the efficiency. Let’s check these out!

Cut to the chase

Yep, we’re saying this again: your email subject line should reflect what your email is actually about. It shouldn’t be long, and it shouldn’t describe your entire message. However, too much intrigue is also a thing that exists — if your email subject is too vague, your subscribers won’t be motivated enough to open it. 

Want to do something intriguing? Give at least a hint, your email is not an “indescribable” Lovecraftian creature, after all.

Here are some good examples we found on Milled:

Keep it short — sort of

Long subject lines take too much time to read and can get awkwardly truncated on mobile devices, so keeping them short is a better strategy. But how short?

According to the recent GetResponse report, subject lines that have 61 to 70 characters yield the highest open rates of 43.4%. The close contender is the length of 21 to 30 symbols, with the average open rate of 43%. These findings are quite predictable — 30 and 70 symbols are the sweet spots that display correctly on mobile and desktop devices accordingly. 

However, what is surprising is that long subject lines get you more clicks — the winners in that category are subjects of 221 to 230 characters. 

What should you do with that data? The general recommendation is still to keep it short for mobile optimization. At the same time, don’t be scared of more descriptive email subjects in appropriate situations.

Ask a question

Let’s say your email promotes a guide for SMM specialists, and its subject line is “STOP doing these mistakes on Reels” or “You’ll NEVER get viral on Reels — here’s why.” While these subject lines have some clickbait value, they make the potential reader feel like they’re in the wrong for having a problem in the first place. 

Don’t turn your subscribers into adversaries — instead, you can ask a question if they need a hand or if they’re struggling. A subject like “Need help with Reels views?” or “Still can’t get a viral Reel?” sounds more empathic, won’t antagonize the reader, and is more inviting to actually open the email.

Give a call-to-action

Imagine your roommate constantly saying things like “I can’t brush my teeth with all the dirty dishes in the bathroom sink” or “The carpet is dusty,” instead of directly asking you to do certain household chores. This is manipulation, and you’d be rightfully pissed off. So why beat around the bush in your subject lines?

Including a clear command in your email subject sets expectations right in the beginning. That way, your subscribers will understand what they need to do and what they will get in return. Here are some nice examples we found on Milled — and the same subject lines rewritten so they don’t have a CTA. Feel the difference!

Personalize wisely

You might think: “Wait, didn’t you say that personalization is out?” — and we actually did. However, the bad kind of email personalization is mindlessly putting first names here and there. Here’s how to personalize email subject lines better:

• Don’t personalize unless the email is personalized. Imagine receiving an email with the subject like “Jenny, this one’s for you!”, opening it, and seeing a generic company news digest that is not even segmented. We would be frustrated! Meanwhile, there’s nothing wrong with the subject itself — it would work wonders for a product recommendation email.
• Use other variables. There’s nothing wrong with first names per se. However, if you want your personalized email subject line to really work, use other subscriber data — location, email, or mobile app engagement, recent purchases, and so on.

For example, this email’s subject line is “Happy 1st Duoversary, Smiles Davis!” Yes, it has the name token — but it’s not the only personalization technique here, the app activity variable is here too. Also, given the email content, this exact personalization makes sense — the email body reveals more personal stats.

Use AI

There aren’t a lot of studies yet that can prove the advantage of AI-generated subject lines compared to human-written ones. Also, some marketers like Margaret Sikora from Woodpecker are not pleased with the output — and we can’t blame them:

However, here’s why you shouldn’t ditch AI for subject lines altogether:

• You don’t have to use AI-generated subjects as they are. AI can be a great tool for brainstorming or simply combating your writer’s block — either way, you always start with something, right?
• You can use AI for better, quicker A/B tests. For example, ask it to come up with other variants for testing based on your “control” subject line and variables, or brainstorm hypotheses.
• It really depends on the tool and the prompt. The article we mentioned above was about testing a bunch of tools specifically for subject lines. Such tools are often underwhelming because prompting may look like filling a pre-built form, and they don’t see your entire email. If you’re unhappy with the output, try, let’s say, conversational AI chatbots like good ol’ ChatGPT — or, if you have an account on Twitter/X — Grok. These AIs are very well-taught, and the conversational UI allows you to write detailed instructions.

Don’t feel like writing a prompt? Try Selzy’s AI subject line and preheader generator! It’s right in your email builder. Once you finish the email, simply open the AI assistant, choose the “Subject line & preheader generator” role, and wait a couple of seconds. 

Instead of trying to understand your prompt, our AI assistant analyzes the entire email, and its output is quite decent! 

The assistant is available in the beta version of the builder. Contact the Selzy support team to get access to it and explore this cutting-edge technology.

Be on brand

As we mentioned earlier, a huge motivator to open your emails is brand awareness. The thing is, a subject line can be your brand asset, just as valuable as your design brand book. Here’s how it works.

Take a look at this screenshot — we removed all the sender information, except for the subject line. You can see all of these are crafted using the same formula: the coffee cup emoji at the beginning, all of the subjects are laconic, give a vague hint about the email content, and sometimes use idioms or very common expressions. Based on that, you can understand that all of these emails are from the same sender — it’s Marketing Brew, by the way.

Same goes for your emails. If you create a consistent style for your subject lines, you’ll be more recognizable in the inbox. That, in its turn, will positively affect brand awareness and, of course, open rates.

Run A/B tests

A bunch of benchmark reports with stats on high-performing words won’t save you from A/B testing — sorry to break it to you! Testing your subject lines is vital because no report or study will tell you what’s going to work on your audience. 

Here’s how to do actually meaningful A/B tests for subject lines:

• Do one variable at a time. Your subject line variants shouldn’t be completely different from each other — they should stay the same except for one thing. That can be using as a question or a regular sentence, with or without an emoji, the emoji placement, the presence or the absence of one personalization token, and so on.
• Keep in mind what exactly you’re testing. Crafting two random variants and setting up an A/B test is not a good strategy. For example, the subjects “Try these SEO hacks” and “SEO hacks to try” are not a good fit for A/B testing. Meanwhile, “Try these SEO hacks” and “Try these 10 SEO hacks” are worth considering for testing, as they explore the influence of numbers.
• Take the results with a grain of salt. ESPs don’t calculate the statistical significance of A/B tests. Meanwhile, it’s an important variable telling you if the pattern you saw is likely to replicate next time. To check it out, use an A/B test calculator — or, if you have the skills, data analysis software like SPSS.

And if you’re interested in creative ideas for A/B tests, check out an issue of FWD: by Selzy.

Email subject line tips for cold emails

Email marketers have it easier — their subscribers are already somewhat warm, so it’s easier to entice them to open another “Take your discount” email. What about cold outreach when you only have one chance of being noticed? In this case, subject lines are particularly important. 

Here are some tips worth considering for sales, outreach, or even a pitch email subject line:

• Don’t make it look like bulk email. By that, we mean generic subject lines like “Article pitch” or “Let’s work together.” Instead, write individual subject lines for every recipient you’re reaching out to. Refer to their website or LinkedIn posts, the person’s position in the company, the company itself or the product they’re working on.
• Think of your intention — and their problem. How can you help the person or the company you’re reaching out to? Are they struggling with something or need a fresh view on their processes? Let’s say, you’re a project manager looking for a new job. If you’re helping improve the existing processes, a subject line like “Need help with your backlog pile?” will be a great choice. But if you invented your own sprint planning method, a subject like “Crunches are a planning problem — I have a solution” is a better choice.
• Add some emotion. Yep, even if it’s B2B cold emails, adding some emotional adjectives like carefree, painless, easy, and so on will help. You need these to create a vision of a perfect world where your prospect’s problem is fully solved and you both live the happiest life possible… And to stand out in a very cluttered inbox, obviously.

Wrapping up

No one likes writing subject lines — rightfully so: it’s hard and a little mistake can cost you many opens, clicks, and conversions. We hope these tips will make it less of an ordeal and more of a fun quest!

Here’s a little recap:

• Cut to the chase — tell your subscribers what your email is about.
• Ask a question — and spark a little curiosity.
• Give a call-to-action — and set clear expectations.
• Keep it short — but if the situation requires, don’t be afraid to get a little more descriptive.
• Personalize wisely — there’s more to personalization than just names.
• Use AI — for writing subject lines, creating A/B test versions, or just for brainstorming.
• Be on brand — and create brand awareness with your email subject lines.
• Run A/B tests — and figure out what works for your audience.