Here's what actually drives opens, clicks, and revenue—and when each approach works best.

A subject line has a single job: Getting as many subscribers as possible to open your email. Focusing on increasing opens will help you identify the most impactful subject line variations for your brand. Plus, a great subject line will improve your email deliverability. After all—if your email doesn't land in your subscribers' primary inbox, they're not going to hear what you have to say.

We analyzed billions of email subject lines to break down the components that can take them from good to great. But remember, a great subject line is a lot like The Beatles: Just as John, Paul, George, and Ringo combined their talents to create greatness, a high-performing subject line is the result of great synergy between the individual components.

Key takeaways 

We analyzed 91B+ subject lines over the last year to find what actually drives opens, clicks, and revenue. The biggest surprise? What works for campaign emails can hurt your triggered email performance. 

• Short subject lines win for campaigns, medium-length works better for conversions 
• Personalization matters in triggered emails but feels out of place in campaigns 
• And here's the real insight: your audience targeting matters more than any subject line trick—recent engagers deliver significantly higher ROI than blasting your entire list

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Why subject lines matter 

Your subject line determines whether your message gets opened or ignored. 

But it's not just about opens—mailbox filters are watching how recipients respond to decide whether your future emails land in the inbox or spam folder. When you consistently send emails people want to open, filters reward you with better deliverability. Get it wrong, and you're talking to an empty room.

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Email subject line best practices 

When crafting subject lines for your emails keep these best practices in mind.

General rules on character count, clarity, relevance 

Since most people check email on mobile, you'd expect shorter subject lines to always win. They do for opens and clicks—but there's a nuance worth understanding.

• For opens and clicks: Short subject lines (under 25 characters) and preheaders (under 30 characters) perform best.
• For conversions: Medium-length subject lines (25-35 characters) and preheaders (30-50 characters) outperform shorter ones. You need enough context to inspire action, not just curiosity.
• Triggered emails: Medium-length subject lines (25-35 characters) drive the most opens. These give shoppers enough context without getting cut off in previews.
• Campaign emails: Short subject lines (under 25 characters) drive the most opens. Jump right into the value—no warm-up needed.

Triggered vs campaign subject lines 

Mailbox filters treat both triggered and campaign emails as marketing, but they recognize that triggered messages result from recent customer action. That helps with deliverability, but the same fundamentals still apply: how recently someone opened, when they last purchased, and how often you've emailed them since signup.

The strategies that work for one type can actually hurt the other. Here's what we're seeing:

Triggered email subject line best practices:

• Product names are the most powerful personalization macro, followed by shoppers' first names: Adding the name of the product a shopper browsed or left in their cart will help you jog shoppers' memories and bring them back to your site. And adding their first name to your welcome series can add a personal touch. You don't want to add their full name to your subject line, though. It's the lowest performer because it'll push the majority of your subject out of the preview.
• Subject lines that include "you" or "your" perform better than those that don't: This goes back to the behavior that triggered the email. "Your cart" makes sense for cart-abandonment journeys—you want to be specific about what action a shopper should take.
• Triggered email subject lines without emojis perform better than those with emojis: Keep your subject line focused on the behavior/action you want to take. These are top-of-mind reminders, not necessarily brand-building moments.

Campaign email subject line best practices:

• Email subject lines that don't include "you" or "your" perform better than those that do: Because campaign messages aren't event-based and are targeted to a broader audience, "you" and "your" won't feel quite as relevant to what you're saying.
• Subject lines without emojis outperform those with them: Subject lines have limited (and precious) space. Emojis add to your subject line's length without providing more information. Plus, emojis can mean different things to different people, muddying your message in such a short space.

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Personalization and when it works 

Personalization effectiveness depends entirely on your email type and who you're targeting.

For triggered emails: Personalization boosts performance, especially with previous purchasers. This audience responds much better to personalized subject lines than first-time visitors. Full name personalization achieves the highest opens and clicks, though un-personalized lines convert most efficiently.

For campaigns: Skip personalization in subject lines. The data shows no meaningful improvement. Save it for the email body.

Previous purchasers outperform never-purchased segments by 13%+, whether you personalize or not.

Emoji use 

Emojis generally hurt performance by taking up space without adding information. Here's what the data is telling us:

• Triggered emails: No emoji plus short subject lines (under 25 characters) achieve the best results across opens, clicks, and revenue. Subject lines that pose a question without emojis get the strongest response.
• Campaign emails: While "$" signs with emojis get high response rates, no-emoji subject lines still win for revenue efficiency. Emojis may be losing their novelty—people might ignore them more than they react to them.

For subject line testing, prioritize human opens to see which lines actually grab attention.

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Audience targeting: The foundation of subject line success

Audiences AI uses real-time customer data to identify subscribers likely to purchase and exclude those unlikely to engage—automatically optimizing your campaign audiences for better deliverability and ROI.

Ultimately, your audience targeting matters more than any subject line tactic. Mailbox filters look at who you're emailing first, then decide inbox placement based on whether you're paying attention to engagement signals.

When 50%+ of your campaign recipients have never engaged or purchased, you're telling filters you don't care about the same things they do—protecting users from unwanted mail. The result? Spam folder.

We see three distinct patterns:

1. High performers target people who opened within the last seven days. This recent interest leads to better inbox placement and strong unique open rates and ROI.
2. Declining performers target people who engaged 6+ months ago. Mailbox providers have told us directly that after 6 months of no engagement, they consider someone "not interested." Any volume over 10% to this segment triggers spam placement. Result: lower opens and ROI.
3. Poor performers send 30%+ of volume to people who signed up but never engaged. This generates the lowest open rates and worst ROI.

Audiences AI helps you automatically identify and target the right segments to maximize deliverability and performance.

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Testing and optimization tips

Use these insights as a starting point, but remember your audience has unique preferences. A/B testing helps you discover what resonates with your specific segments.

When testing subject lines, change only one variable at a time—like including a product name versus not—without altering other copy. This helps you isolate what actually drives performance.

What to measure: Focus on unique open rate, specifically human opens rather than machine opens, to see which subject lines truly grab attention.

Human opens tell you someone was genuinely interested in your subject line. Machine opens confirm your message landed in the inbox (tracking pixels don't fire from spam folders), which is useful for understanding deliverability.

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Metrics and how to measure success

Track these KPIs to evaluate subject line performance:

• Unique open rate: Focus on human opens (not machine opens) to measure genuine interest in your subject line. This is your primary indicator of subject line effectiveness.
• Click-through rate (CTR): Measures how compelling your subject line is at driving action beyond the open.
• Conversion rate: Shows how effectively clicks convert to purchases. Medium-length subject lines (25-35 characters) typically win here.
• Revenue per send/open/click: Evaluates efficiency. No-emoji subject lines and recent audience targeting drive the best revenue efficiency metrics.
• Machine vs human opens: Machine opens confirm inbox placement; human opens confirm interest. Both matter, but prioritize human opens for subject line testing.

Subject line changes directly impact these metrics—even small tweaks to length, personalization, or language can shift performance significantly based on whether you're sending triggered or campaign emails.

Ready to take your email testing further? Discover how to optimize your campaigns for both machines and humans to maximize deliverability and conversions.

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Common mistakes to avoid

• Using misleading tactics like "RE:" or "your order confirmation" for promotional emails. These trigger spam filters now.
• Mismatching subject and content. Claiming "80% off" when products are full price leads to spam complaints and unsubscribes.
• Using emojis in triggered emails. They reduce performance by taking up space without adding context.
• Over-personalizing campaigns. Personalization feels forced in broadcast emails and wastes characters.

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Frequently asked questions 

1. How long should my subject line be? It depends on your goal. For maximum opens and clicks, keep it under 25 characters. For best conversions, use 25-35 characters to provide enough context to inspire action.
2. Do emojis increase email opens? No. Subject lines without emojis consistently outperform those with them, especially in triggered emails. Emojis take up space and seem to be losing their novelty—people ignore them more than react to them. The only exception: campaign emails with "$" signs and emojis show strong response rates, though no-emoji still wins for revenue efficiency.
3. Should I personalize all subject lines? No. Personalize triggered emails, especially for previous purchasers, but skip personalization in campaign subject lines. Focus on sounding like your own brand—your subject lines, website, and app should all match in tone and wording.
4. What's the best discount to offer? 40% discounts are most efficient, generating 12% of discount-offer revenue while representing only 9.8% of volume. Subject lines with "$" amounts outperform "%" signs in campaigns. Overall, discount subject lines make 24% of revenue while representing 22% of volume—punching above their weight.

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Ready to grow and optimize the rest of your email program? Explore our email marketing hub for strategies on delivering better experiences and consistent messaging across channels, or dive into these specific resources:

• Tips for making sure your emails land in subscribers’ primary inbox 
• How you can best combine your SMS and email marketing programs 
• Segmentation best practices to help you deliver more personalized experiences