How to Write Stronger Subject Lines
Generated by Klaviyo AI
Effective email subject lines are concise, contextual, personalized, and designed to create relevant curiosity. Because Apple Mail Privacy Protection can inflate open rates, subject line tests should distinguish privacy opens from true engagement.
- Personalize with context: Use customer behaviors and preferences rather than relying only on first names.
- Keep it short: Write mobile-friendly subject lines and use preview text to add detail or hint at a solution.
- Create relevant curiosity: Match the curiosity gap to audience intent and resolve it clearly in the email.
- Use tactics carefully: Apply urgency, scarcity, capitalization, and a maximum of one emoji only when they suit the audience and message.
We’re obsessed with subject lines. And you should be, too.
Without a persuasive subject line that piques curiosity, your email body copy can’t do the work it was destined to do. No opens means no clicks, and no clicks means no revenue.
We’ve scoured through some of the best email subject line examples from our customers to reverse-engineer best practices and come up with some first principles to help you craft and test subject lines and drive higher open rates.
Before you start: Apple Mail Privacy Protection and email tracking in iOS 27
We can’t talk about subject lines without first addressing how they’re measured: open rates.
Apple Mail Privacy Protection (MPP) is still active for iOS 27, launched in September 2026. That means Apple still hides the recipient's IP address from senders and prevents those senders from tracking whether recipients opened a message.
Considering Apple owns 62.26% of email client market share as of July 2026, that means well over half of your brand’s open rate metrics don’t reflect how many people actually opened your email.
With that in mind, Ananda Farge, former associate director of email and SMS marketing at Accenture, says she still values open rate, but only relative to itself and other metrics that inform her about the overall health of an email program.
“We know that a 60% open rate doesn’t mean 60% of people actually opened your email,” Farge says. “But if we had a 60% open rate on Monday, and on Tuesday we see a 40% open rate, we can compare campaigns and make decisions from there. Maybe deliverability dropped off, so we’ll look at that.”
More practically, the best way to manage this reality is to use your CRM to differentiate MPP from true recipient engagement. The purpose of this is to avoid seeing inflated email open rates that may lead you to believe your subject lines are more effective than they really are.
Klaviyo handles this with a property called “Apple Privacy Open,” which is either true or false for all open events. If Apple Privacy Open is true, it means the message was opened on a device with MPP turned on. That means the open may be attributable to Apple’s MPP and doesn’t necessarily reflect a true email open.
The next step is to separate MPP opens from true opens by adding a filter where Apple Privacy Open equals false. This is the segment to test subject lines with, because you know an open from this segment is actually an open.
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“This will be a smaller segment than what you’re used to,” points out Dayna Scandone, senior digital strategist at Klaviyo. “If the email marketing campaign is important, you can always follow up with a second campaign to everyone else and use Smart Sending to ensure no one gets the campaign twice.”
8 email subject line best practices for boosting open rates
With all that in mind, here are 8 email subject line first principles and best practices to test for your own brand and audience:
1. Personalization makes email subject lines stand out in the inbox
Generally speaking, the larger the company, the more likely they are to use personalization in email subject lines. Klaviyo research finds that mid-market companies, for example, tend to personalize 3.35% of their email subject lines with the recipient’s first name, while small to midsize businesses only do so in 2.45% of their email subject lines.
Much of the time, “personalization” in a subject line means using the reader’s first name, but our research suggests first name alone won’t do much to increase your open rates. Personalization is actually a much deeper concept, in that your subject lines need to holistically reflect your customers’ unique behaviors and preferences.
The best way to achieve deeper personalization with your subject lines is to:
- Centralize all your customer data, from every customer-facing platform, in one B2C CRM. With all your data points about every customer in one shared profile, you’ll have more options to draw on for your subject lines because you can come at personalization from as many angles as there is customer data.
- Use a CRM that combines customer data with agentic AI, customer service, analytics, and marketing automation functionality. When all of these activities are happening in the same place, your data is more likely to be clean and up to date, resulting in more accurate, relevant messaging.
- Create emails, including subject lines, with an AI marketing agent built into your CRM. That way, your AI agent can automate subject line personalization with access to all the customer data it needs to personalize your content.
2. Email subject lines perform better when they’re short
According to our research, as subject lines get wordier, subscribers on your email list are less likely to open your emails.
“Most email opens are happening on mobile devices, so it’s critical to think mobile-first when you’re writing subject lines,” Scandone points out. “You should always be testing your campaigns on desktop and mobile to ensure they’re showing up exactly how you want them to, and that includes the subject line.”
So, how long should your subject lines be? On average, subject lines across all business sizes are about 7 words long (including emojis). According to our data, subject lines of this length drive open rates of about 30%.
If you want to challenge yourself to shorten your subject lines even more and potentially boost open rates, you can lean on another important tool for piquing curiosity: preview text.
3. Thoughtful email preview text can promise a solution to a problem
We can’t talk about subject lines without mentioning the importance of preview text.
“Solving a pain point in your subject line can work well to encourage email opens,” says Toccara Karizma, CEO of Karizma Marketing. One of the best ways to do this is to present it in your subject line and hint at a solution in your preview text.
4. Urgency and scarcity prompt opens, but proceed with caution in email subject lines
Limited-time offers, exclusive offers, low-stock messaging, and clearance sales are all about creating urgency and scarcity: two things that inspire people to act. The most obvious way to create a sense of urgency is using phrases like “limited time” and “ending soon.”
A word of caution: While these kinds of words don’t have the same detrimental effect on email deliverability as they used to, we still recommend careful and appropriate use based on your audience to make sure your email doesn’t end up in the spam folder.
Whereas an audience segment with low purchase intent might find your use of urgent language, exclamation points, and all caps annoying, a segment with high buying intent or purchase history may welcome it.
5. Context rules all good content performance of email subject lines
“Consider the context [of your email],” says Nikki Elbaz, an email copywriter and retention specialist. “By context, I mean the inbox. What kinds of emails does your subscriber usually receive? Will emojis/questions/whatever style subject lines you’re writing stand out or blend in?”
Most email marketers understand context, in theory. They know their emails are showing up in flooded inboxes, received by overwhelmed, busy people. But hyper-personalization is how you can overcome some of this information overload.
When it’s drawing from real-time customer data, your AI marketing agent can likely offer up context signals you may have overlooked because they’re latent in your data. This is because your AI agent can analyze more customer data than humans can in a shorter period of time.
In a conversational prompt, try asking your AI agent for suggestions on how to personalize your subject lines for sub-segments within your intended audience. You’ll at least get recommendations you can test, which will likely lead to better contextual subject lines after a few trial rounds.
6. Most great email subject lines create a curiosity gap
The curiosity gap is the “space between the information we’re given (what we know) and the information that’s initially withheld (what we don’t know),” according to Enchanting Marketing. This is what you want to create with your email subject line‚ because opening your email is how your audience closes the gap.
Curiosity is especially important if you’re hyping a new product, collection, or campaign that hasn’t launched yet. But keep in mind that what you spark curiosity about will depend on the purchase intent of your audience segment.
In the early stages of an email subscriber relationship, you may want to spark curiosity about the more salient points of your brand story or the problems you solve. But after you’ve established a relationship and subscribers are indicating higher purchase intent, through website visits, abandoned carts, etc., that’s when you might want to spark curiosity about your actual product releases.
A word of caution: Just as you wouldn’t enjoy reading a novel without a resolution, email recipients don’t appreciate a curiosity gap that stays open. If you’re going to pique curiosity in a subject line, make sure your email copy keeps its promise by satisfying that curiosity.
7. Emojis aren’t bad in email subject lines, but use them sparingly
If you’re trying to stand out in a crowded inbox, you might think emojis are a good way to get noticed. But you may be blending in more than you realize: according to Klaviyo research, 42% of mid-market businesses and 43.5% of SMBs use emojis in their subject lines.
Here are some quick tips for using emojis in subject lines:
- Don’t use more than one.
- Use emojis to amplify the message, not replace words.
- Remember that the goal of the subject line is to deliver a clear message. If an emoji helps you do that, go for it.
A word of caution: Emojis show up differently depending on the inbox provider, so make sure to preview your subject lines across inbox providers and devices.
8. Auditing your subject lines with AI can reveal insights you’d never find otherwise
As part of your flow audit, ask your AI marketing agent to focus on open rates to zero in on subject line performance. Within the same AI conversation, you’ll be able to get a sense of your top underperforming email flow subject lines and get suggestions for how to improve them.
Your AI agent should be able to build the marketing work that follows from the recommendations, so you can just go ahead and launch your new subject lines without a delay.
9 email subject line examples that stand out in inboxes
1. Supply uses an emoji to convey context
Subject line: This email is better than 👇
It’s one thing to understand that inboxes are overflowing, but it’s another to use that fact to your advantage with your subject line.
In this subject line example, shaving brand Supply makes damn good use of an emoji to show they understand the context of the average inbox. With solid reinforcement from their preview text, Supply crafts one of the most creative yet simple subject lines we’ve ever seen.
2. Dagne Dover prioritizes mobile with a punchy subject line
Subject line: To you, from you
You don’t necessarily need a super short subject line to see high email open rates. Remember that the average word count is 7, according to our research. But at the very least, your subject line should look great on mobile.
Dagne Dover, the brand that creates “problem-solving bags for problem-solving humans,” demonstrates a mobile priority with this subject line that’s just 4 words long. It stands out in an inbox not only because it’s shorter than other subject lines, but also because it’s so concise and direct.
Dagne Dover
3. Beekman 1802 builds urgency for a free offer
Subject line: LAST CALL 📣 Free Mini Milk Drops
All caps, an emoji, and a freebie available for a limited time: these are the ingredients of a subject line that inspires a sense of urgency.
Assuming skincare brand Beekman 1802 sent out a couple emails before this one to communicate the offer, this is a great subject line for everyone on the email list who didn’t buy during previous rounds.
Beekman 1802
Email subject line tip: If you’re sending last chance reminders like this, you’ll want to exclude people in the segment who already took action from previous emails in the series.
And consider experimenting with other channels besides email. Because people are on their phones more than in their inboxes, last-chance messaging tends to work well on mobile channels like text messaging and WhatsApp. Use AI-powered channel affinity to automatically route last-chance messages to the channel where each subscriber is most likely to engage next.
4. Dossier creates a curiosity gap for customers
Subject line: ✨Something NEW is coming…✨
If you’re sending an email to customers, especially VIPs, what better way to hook them than with something new?
Fragrance brand Dossier evokes curiosity with this simple, clear subject line. It may not work on people who don’t have high purchase intent, but on the flip side, it could work really well on people who have bought before.
Dossier
Email subject line tip: This is why it’s so important to use an AI marketing agent built into your CRM. It can untangle the customers who would likely respond well from the ones who wouldn’t, and make sure everyone gets the right version.
5. Reverie’s subject line and preview text go hand in hand
Subject line: Are you washing your hair properly?
Preview text: How to do it right
In this subject line example, haircare brand Reverie hints at solving a pain point by asking a provocative question and then expanding on it with their preview text. It’s a great example of how to keep your subject line short while using the extra real estate of your preview text to widen the curiosity gap.
Reverie
6. House of Harlow plays into celebrity
Subject line: Shining on Chase Infiniti ✨
In this subject line example, jewelry brand House of Harlow invokes the name of Chase Infiniti, up-and-coming celebrity and star of the dystopian drama series The Testaments, to draw recipients in. The subject line opens a curiosity gap that’s immediately closed when the recipient opens the email and sees which piece of jewelry Chase Infiniti prefers to wear with her stunning red dress.
Email subject line tip: If you have a high-quality image you’d love to feature as the focal point of your campaign, ask your AI marketing agent to build an offer and messaging around it.
7. Fly by Jing demonstrates empathy for tough holidays
Subject line: Want to opt out of Father’s Day?
In this subject line example, sauce and seasoning brand Fly By Jing thinks ahead of Father’s Day by thinking about people who may not want to celebrate it. With a subject line that hits home for many, the brand pulls people in with the most important marketing attribute: empathy.
8. OSEA gets personal with a unique discount nudge
Subject line: Your 10% Off Ends Soon!
Vegan skincare brand OSEA Malibu draws people in with the most powerful subject line tactic there is: a personalized discount.
OSEA Malibu
Email subject line tip: A personalized discount strategy is just as good for your brand as it is for your email recipients. When offering discounts, you want to make sure you’re not offering them to people who would buy at full price.
Consider using predictive analytics to reduce discount waste. For example, you can use churn risk prediction in combination with expected date of next order to better understand whether you should be targeting someone with a win-back discount because they’re about to churn, or whether someone is a loyal customer who would probably buy at regular price.
9. Wildfang puns their way to curiosity with an inside joke
Subject line: Overall, we’re obsessed
In this email subject line example, queer apparel brand Wildfang uses a pun to charm their way into an open while piquing curiosity as to what exactly they’re obsessed with.
Email subject line tip: This is a subject line that could work particularly well for loyal customers. Shoppers who frequently buy from Wildfang are aware the brand is known for their high-quality overalls, so they would likely get the joke right away.
If your brand can get a similar in-joke across in your subject line, it could be a great way to make your loyal customers feel like they’re part of a community.
What’s next: personalize your email subject lines with an autonomous B2C CRM
Your email subject lines are only as good as how well you’re able to personalize them, and your personalization is only as good as your customer data.
Klaviyo B2C CRM, the autonomous CRM for B2C, houses all the customer data you need to personalize thousands or even millions of subject lines at scale.
And with Composer, the always-on AI marketing agent built into Klaviyo, you can create new emails in minutes. Start with a goal, a brief, or something Composer uncovered in your customer and performance data. Composer can help build the audiences, flows, and campaigns across email, text message, and mobile push, using your brand and business context throughout the process.
Ideate, write, and optimize better subject lines with Klaviyo.
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