TL;DR: Most email subject lines are overthought and under-strategized: a room full of opinions, 20 ChatGPT options, and a pick made on vibes. The top 9- and 10-figure brands use a system instead. My FLIGHT Check runs every subject line through six rules: Focus on the customer, Lead with value, Intrigue them, Give a reason to act, sound Human, and keep it Tight. Once a subject line passes all six, A/B test three versions and let the data pick the winner.
Book a Free Retention Audit To See The Full Revenue Potential of Your Customer List >
Key Takeaways
- Nobody can click or buy from an email they never opened, so the subject line matters more than any other part of the email.
- Top brands don’t ask for the perfect subject line. They ask whether it has the ingredients to earn an open from every subscriber, one-to-one.
- The FLIGHT Check has six rules: Focus on the customer, Lead with value, Intrigue, Give a reason to act, sound Human, and keep it Tight.
- Pick the segment first, then write the subject line. “Back for another pair of runners? Try these” beats “Our running collection has arrived.”
- Know your limits: about 37 characters in the Gmail iPhone app, 48 in Apple Mail on iPhone and 70 in Gmail desktop. Put the value first.
- Write three subject lines that pass the FLIGHT Check, A/B test them on 50% of the list and send the winner to the rest. Data is the tiebreaker.
- This system helped one client grow email revenue from $47M to $73M a year in eight months.
Get my $500M Black Friday & Cyber Monday Gameplan — free. My complete Q4 campaign calendar, checklists, videos and playbooks, built from $510M+ in client revenue. Grab it at BlackFridayPlaybook.com >
Here’s a scene I’ve watched play out at brands of every size.
The email is done. The design looks great, the creative is sharp, the linking strategy is set and the offer is locked. Then someone asks, “What’s the subject line?”
And the opinions start. The designer has one. The strategist has one. The CMO has one. The CEO has one. Nobody agrees, so somebody asks ChatGPT for 20 subject lines, the team picks one on vibes, and they hit send.
That’s not a strategy. It’s a guessing game.
The problem is that the subject line decides whether the email gets opened at all. Forget clicks. Forget conversions. Nobody can click or buy from an email they never opened. Get the subject line wrong and all the work that went into the design, the offer and the links never gets seen.
I’ve sent more than 4 billion emails and run thousands of subject line tests, first as VP of Retention at 10-figure retailers and now for the 180+ D2C brands and retailers we’ve worked with at HiFlyer Digital. In this post I’ll walk you through the six-rule framework I use to take the opinions out of email subject lines. It’s the same system that helped one of our clients grow from $47 million to $73 million a year in email revenue in just eight months, driven by higher opens that led to more clicks, more conversions and more sales.
Why Do Most Email Subject Lines Fail?
Most email subject lines fail because they come out of opinion instead of a system. Search “subject line best practices” and you’ll get a pile of contradictions:
- Keep it under 40 characters. No, go longer.
- Use emojis. Never use emojis.
- Use the first name. Don’t use the first name.
- Send from the brand. Send from the founder.
- Ask a question. Build intrigue. Be direct.
Most of those “rules” come from someone’s experience on one or two brands. With so many opinions out there, how do you know which ones are right?
The top brands have the resources, tools, testing infrastructure and team to find out what actually works. And they don’t ask, “What’s the perfect subject line?”
They ask a better question:
“Does this subject line have the ingredients it needs to earn an open from every subscriber on our list, and how do we connect with each of them one-to-one as often as possible?”
My whole philosophy comes down to one idea: there’s a person behind the inbox. My job as an email marketer is to connect with that person. A framework makes that repeatable, for campaigns, for flows and even for SMS.
Where Did the FLIGHT Check Framework Come From?
I had a head start on subject lines. Back when I was running retention programs at 9- and 10-figure retailers, years before ChatGPT, we worked with Persado, an AI-powered subject line platform. Persado built subject lines around emotion: gratitude and appreciation for your best customers, exclusivity for VIPs, urgency for customers who weren’t engaging, plus newness, scarcity and limited-time language. The AI matched the right emotion to the right type of customer.
That taught me something important. The top brands don’t just use the right tools. They have a framework for finding the best subject line for every subscriber, one-to-one, on every device.
After years of trial and error across 9- and 10-figure brands, I boiled that framework down to six checks. I call it the Subject Line FLIGHT Check, because an email doesn’t take off until it’s opened.
What Is the FLIGHT Check System for Email Subject Lines?
The FLIGHT Check is a six-rule checklist every subject line has to pass before it ships:
- F: Focus on the customer. Make it relevant to the right customer, behavior or moment.
- L: Lead with value. Give a clear reason to care about what’s inside. Don’t bury the lede.
- I: Intrigue them. Open an information gap that makes them want to find out more.
- G: Give them a reason to act. Use urgency, scarcity or timeliness so they open now.
- H: Human, not corporate. Sound like a real person, not a marketing department.
- T: Tight and easy to scan. Cut extra words and make sure it works on every device.
When a subject line passes all six checks, you’ve taken most of the subjectivity out of the process. You also get something the team can agree on. You have an opinion, I have an opinion, the founder has an opinion. So whose opinion wins? At the top brands the answer is simple: data is the tiebreaker. The FLIGHT Check gives you strong candidates, and testing tells you which one wins.
Let’s break down each rule with real before-and-after examples from inboxes.
Rule 1: How Do You Focus a Subject Line on the Customer?
Focus your subject line on the customer by choosing the segment first and writing the subject line for that segment. Most brands design the campaign first and pick the audience second. The top brands do it the other way around.
This comes straight from my four-pillar retention strategy: segmentation, personalization, automation and multiplication. The first two pillars carry every high-performing email. Right person, right message, and it starts with the right person.
Here’s a subject line I got from a running shoe brand:
“Your weekend miles start here” 👟
It’s vague, and it isn’t talking to me. Say I’m a VIP who has bought from you five or six times and browsed running shoes yesterday. How does “Your weekend miles start here” connect with me? It doesn’t. That’s a one-size-fits-all subject line sent to check a box. Another brand sent “Our running collection has arrived.” That’s nice, but where’s the customer?
Now write it for that VIP who was browsing runners:
“Back for another pair of runners? Try these”
- “Back” tells a repeat customer you know them.
- “Another pair of runners” matches what they browsed yesterday.
- “Try these” adds intrigue. Which ones? Open to find out.
The takeaway: build the segment first, then write the subject line and design the campaign for that segment.
Rule 2: What Does “Lead With Value” Mean in a Subject Line?
Leading with value means your subject line states the benefit up front instead of burying it behind clever copy. I picked this up working in media and publishing. The best newspaper and magazine writers take a 300-word or 5,000-word article and pack the main point into six to eight words of headline. That’s what “don’t bury the lede” means, and a subject line is your headline.
Subscribers are scanning for a few things:
- Are you saving me money?
- Are you saving me time?
- Are you solving a problem?
- Are you giving me early access, exclusivity or something new?
If the value is pushed to the end or left vague, you’ve buried the lede and the email doesn’t get opened.
Here’s a real one from a cookware brand running a plastic trade-in with a $100K giveaway:
“Swap out the plastic. Double your entries for 100K” (50 characters)
On my iPhone the “100K” gets cut off. The biggest hook in the email, a six-figure giveaway, is the part the subscriber never sees. That’s literally burying the lede. Three better versions:
- “100K to swap out plastic?” (25 characters): the value is up front and the question mark adds intrigue.
- “Ditch plastic, double your 100K entries” (39 characters): about six words, straight to the point, and it reads on every device.
- “Goodbye plastic, hello 100K” (27 characters): clear value, and it makes you curious how the two connect.
Rule 3: How Do You Create Intrigue Without Clickbait?
You create intrigue by opening a real information gap, something true that the subscriber can only close by opening the email. The subject line has one job: get the open. We’re not worrying about clicks or conversions yet. But when opens go up, clicks go up, conversions go up, and you make more money.
What you don’t want is a head fake:
- “Open for a free gift” when the gift is another 10% off coupon
- “Surprise, this is for you” with nothing behind it
- Fake “Customer service” or “Order confirmation” subject lines on a promo email
Head fakes pile up complaints. The email team may never see those complaints because they don’t run customer service, but your brand feels every one of them.
Good intrigue looks like this subject line I got from a supplement brand bringing back a lemon-raspberry flavor:
“The flavor that never should have left”
It’s short and it raises two questions: what flavor, and why did it leave? Here are more that open a real gap:
- “The product we can’t keep in stock”
- “Everyone keeps buying this one”
- “We weren’t expecting this to sell out”
Each one is true, specific enough to be believable and incomplete enough to earn the open.
Rule 4: How Do You Give Subscribers a Reason to Act?
You give subscribers a reason to act by building urgency, scarcity or timeliness into the subject line. Your subscribers get hundreds of emails a day, and you probably sent them one yesterday and will send another tomorrow. So why should they open this one right now?
- Urgency: ends soon, last chance, selling out
- Scarcity: only 17 left, limited release
- Timeliness: best of the month, new arrivals, starts today
That’s FOMO: “I need to act on this now or I’ll miss out.”
Here’s a real subject line from a major beauty retailer:
“Get up to 50% on select beauty and get 500 bonus points with $75 spent” ❤️ (70 characters)
Set aside the missing “off.” That’s a lot to unpack, and it gets clipped in the Gmail app and Apple Mail on iPhone. Meanwhile the creative said “Starts today” and “Stacking weekend.” The reason to act was in the email, just not in the subject line.
That’s why I always review the subject line and the creative together. When we test subject lines, we track clicks as well as opens, because if the open was honest, the creative should get the click. Here’s how I’d rewrite it:
- “Weekend only. 50% off beauty” (28 characters)
- “Starts now. Weekend sale.” Four words, and both carry urgency.
- “48 hours, 50% off, plus secret bonus” (36 characters): covers everything they wanted to say. “Secret bonus” stands in for the 500 points and adds intrigue.
You don’t need to cram every detail into the subject line. Let the email do its job.
Rule 5: How Do You Make a Subject Line Sound Human?
Run the human test: would a real person actually say this? Anyone on your team can run it, from the designer to the marketing coordinator, because everyone on your team is a human.
The worst offender is the fake founder email. I reply to every founder-led email I get. One came in from a CEO: “Isaac, thank you for joining our list… here’s why I built this brand… and here’s 10% off your order.” So I replied: “Hey Jim, good to meet you. I have a few questions about your brand. You got a minute?”
He never answered, because it never went to the CEO. Nobody was reading it. That’s a head fake, and it’s embarrassing for the founder whose name is on it.
More subject lines that fail the human test:
- “Hi, I want to share something meaningful with you”, on what was clearly a 20%-off sale email from a home medical brand
- “There, this might make things easier”, where the first-name merge tag failed and fell back to “There”
- “Dear customer”. This isn’t the 1960s. Nobody talks like that.
Rewrite them the way people actually talk:
- “New subscribers like you love these”
- “Isaac, looking for this?” For a missing first name, fall back to “Hey, looking for this?” or “FYI, looking for this?”
- “Found for you: [Product Name]” for browse and cart abandonment. For example, “Found for you: Sony A6400” or “Found for you: Apple MacBook.” You know what they looked at, so say it.
If a person wouldn’t say it, change it.
Rule 6: How Long Should an Email Subject Line Be?
Keep the key message inside the shortest limit your subscribers will see it on. Know the character limits for each device:
| Inbox / Device | Approx. visible subject line characters |
|---|---|
| Gmail app (iPhone) | 37 |
| Apple Mail (iPhone) | 48 |
| Gmail (desktop) | 70 |
Here’s a real subject line from a work-shoe brand:
“Your fall shift starts now, 25% off slip resistant shoes – ends soon” (68 characters)
On my iPhone it read: “Your fall shift starts now, 25% off slip re…” The product and the “ends soon” urgency were both cut off. It buries the lede and it isn’t tight, which breaks two rules at once. The fix:
- Put the key information first. 25% off and “ends soon” matter more than “fall shift.”
- Cut every unnecessary word.
- Use emojis when they fit the message. An emoji can replace a word, save characters and add emotion. A 🎁 on “Surprise, we have a gift for you” or a 👉 pointing at the product works because it matches what you’re saying.
What Does a Subject Line That Passes the FLIGHT Check Look Like?
Let’s run the full check on a real email from an 8-figure B2B industrial brand. Every month they remind loyalty members to redeem rewards before they expire. The subject line:
“Matthew, your 1,284 points are expiring”
The first name and the point balance are both dynamic for each customer.
- ✅ Focus: It’s personalized with his name and his balance, and it talks to one person.
- ✅ Lead: Nothing is buried. There’s no fluff like “Your rewards update.” The message is his points, and they’re expiring.
- ✅ Intrigue: Matthew doesn’t care about 1,284 points. He cares what they’re worth in dollars. Is it $2? $20? $200? He has to open to find out. And here’s the hack: the creative matches the subject line. The email shows his points with a CTA that says “See how much that is,” which sends him back to the site. The intrigue carries him from the subject line to the click.
- ✅ Give: “Expiring.” When? Today? Next week? Next month? That’s his reason to open now.
- ✅ Human: It’s helpful and relevant, like a friend telling you your car warranty is about to expire. Nothing about it reads like it came from a machine.
- ✅ Tight: 39 characters, and fewer for shorter names. “Isaac” brings it down to 37. The main message shows on essentially every device.
Six for six. That’s a subject line ready for takeoff.
Should You Still A/B Test Subject Lines That Pass the FLIGHT Check?
Yes. Write three subject lines that pass the FLIGHT Check and A/B test them. Send the test to 50% of the audience, then send the winner to the remaining 50%.
Even though these are the rules the top brands follow, trust but verify on your own list. Every audience is different, and every test teaches you something. Once a subject line and campaign prove themselves, automate them so the winner keeps working without you rewriting it every time. That’s the automation and multiplication pillars doing their job.
Want the shortcut? I built a full AI prompt that runs the FLIGHT Check for you. It’s in my AI for Email Playbook, along with the prompts we use for subject lines, imagery, creative and linking strategy. Get it free at hiflyerdigital.com/playbooks.
How Does the FLIGHT Check Apply to Black Friday Subject Lines?
BFCM is where the FLIGHT Check matters most. Your subscribers’ inboxes are more crowded that week than at any other time of year, and every brand is shouting “50% off.” The subject lines that win in Q4 are the ones that:
- Focus on segments: VIPs get early access, lapsed buyers get a reason to come back, and new subscribers get a first-purchase offer.
- Lead with the offer in the first 37 characters, not at the end.
- Give real, honest urgency like “Ends tonight” or “Early access: 24 hours,” with no fake countdowns.
- Stay Tight, because most BFCM opens happen on mobile.
Run every Black Friday and Cyber Monday subject line through the six checks before it goes into your send calendar.
The Bottom Line
Subject lines are the most important part of any email, because nothing else happens without the open. Stop letting opinions and vibes pick them. Here’s what to take away:
- Remove the opinions. Don’t let subjectivity, or a random list of AI ideas, write your subject lines. Use a proven framework.
- Run the FLIGHT Check: Focus, Lead, Intrigue, Give, Human, Tight.
- A/B test three FLIGHT-checked versions and let the data be the tiebreaker.
Do that on every campaign and flow, and you’ll get higher opens, which lead to more clicks, more conversions and more sales.
Frequently Asked Questions
What is the FLIGHT Check for email subject lines?
The FLIGHT Check is Isaac Hyman’s six-rule framework for writing email subject lines. A subject line has to Focus on the customer, Lead with value, Intrigue the reader, Give a reason to act, sound Human and stay Tight enough to scan on any device. It takes the opinions and guesswork out of choosing subject lines.
How long should an email subject line be?
Keep the core message within about 37 characters, which is roughly what the Gmail app on iPhone shows. Apple Mail on iPhone shows about 48 characters and Gmail desktop about 70. Put the offer or value first so nothing important gets cut off.
Should I use emojis in email subject lines?
Yes, when the emoji fits the message. An emoji can replace a word, save characters and add emotion, like a gift emoji on a gift offer. Don’t add emojis just for decoration.
Does using the subscriber’s first name improve open rates?
Personalization helps when it’s part of a subject line that’s actually relevant, like “Matthew, your 1,284 points are expiring.” Always set a natural fallback such as “Hey” or “FYI” so a failed merge tag never produces “There, this might make things easier.”
What makes a subject line spammy or clickbait?
Head fakes like “Open for a free gift” when there’s no gift, or fake “Order confirmation” subject lines on promo emails. They may get an open once, but they drive complaints and erode trust. Real intrigue opens an honest information gap, like “The product we can’t keep in stock.”
How should I A/B test email subject lines?
Write three subject lines that pass the FLIGHT Check. Send them to 50% of your audience, then send the winner to the remaining 50%. Track clicks as well as opens so you know the winning subject line led people to the creative. Automate proven winners in your flows.
Want expert help with how to write email subject lines that get opened? See how we do it for brands like yours.
If you want to see how your email program stacks up and what it would take to improve it, you can start with a free audit:
Book a Free Email & SMS Audit & Discover The True Revenue Potential of Your List >
Get my $500M Black Friday & Cyber Monday Gameplan — free. My complete Q4 campaign calendar, checklists, videos and playbooks, built from $510M+ in client revenue. Grab it at BlackFridayPlaybook.com >
Loved this blog post? Take one of these next steps:
- Get the FREE $500M Black Friday & Cyber Monday Gameplan
- Get a FREE Email Marketing & SMS Audit
- Get our Email & SMS Playbook
- Grab my MachX Email Masterclass
- Subscribe to our Youtube Channel
- Connect with Isaac on LinkedIn
- Follow us on Instagram
- Book a 15 Minute Discovery Call