AI for Email Subject Lines
You’ve written the email. The content is done, the link is in, and then you hit the subject line field and freeze. Using AI for email subject lines is how I get past that moment, and it’s what I’m going to walk you through here.
I’m not going to hand you a “this trick doubled my open rate” story. My blog is still young and my email list is small, just a handful of sign-ups so far. What I do have is a real, repeatable process that I use on every email, and it works for someone starting from zero. Starting small is also the best time to build good habits, because you’re not fixing bad ones later.
Why a Blank Subject Line Field Is Its Own Kind of Writer's Block
Most people think the body of the email is the hard part. It’s usually the subject line.
Here’s why. The body of an email has room. You can ramble a little, find your point, and edit. The subject line has maybe six to ten words to do one job: convince a busy person to open something. Every word carries weight, and that pressure is what freezes people.
There’s also a trap that catches a lot of beginners. You spend an hour on the email, then rush the subject line in thirty seconds because you’re tired. That’s backwards. The subject line is the only part every single subscriber sees. If it fails, no one reads the rest of your email.
This is where AI earns its place. It doesn’t replace your thinking. It gets you out of the blank-page freeze by putting options in front of you, and picking between options is far easier than inventing from nothing.
What Actually Makes a Subject Line Get Opened
Before you use any tool, you need to know what you’re aiming for. Otherwise, you can’t judge what the AI gives you. These are the principles I work from.
1. Clarity beats cleverness
A clever subject line that confuses people gets ignored. A clear one that tells the reader what’s inside gets opened by the people who want it.
Compare these two made-up examples:
- “The thing nobody tells you”
- “3 free tools I use to schedule blog posts”
The first is mysterious, but it could be about anything. The second tells the right reader exactly why to open. When you’re new and your list is small, those readers are the ones you want. You’re not trying to trick everyone into opening. You’re trying to reach the people who’ll care.
2. Curiosity works, but only when you pay it off
A little curiosity is useful. A subject line like “The mistake I made with my first opt-in form” makes people want to know what it was. That’s fine, as long as the email delivers.
Clickbait is curiosity with no payoff. If the subject line promises a secret and the email contains a normal tip, the reader feels tricked, and next time they won’t open. That damages trust, and trust is the whole asset you’re building with a list.
My rule: if I couldn’t comfortably say the subject line out loud to the reader’s face and then deliver what it promised, I rewrite it.
3. The subject line has to match the email
This one sounds obvious, but it’s where a lot of AI-written and rushed subject lines go wrong. The subject line makes a promise, and the email has to keep it. A mismatch drives unsubscribes and spam complaints, which hurt you more than a lower open rate does.
4. Keep it short enough to read on a phone
Many people check email on their phones, and long subject lines get cut off. I aim to put the important words first so that even a truncated version makes sense. There’s no magic number, but if you’re past roughly ten words, ask yourself what can go.
5. Avoid the usual spam triggers
You probably already know the obvious ones: all caps, lots of exclamation marks, “FREE!!!” and pushy urgency. Spam filters and readers both react badly to them. Brevo, the email platform I use for my opt-in pop-ups, has its own help article on writing great email subject lines, and it’s worth reading alongside this one for the platform’s view on what works.
Where AI Fits In (and Where It Doesn't)
Let me be direct about what AI is good at here and what it isn’t, because both matter.
What AI does well:
- It produces many options quickly, so you never face a blank box.
- It shows you different angles on the same email: a how-to angle, a curiosity angle, a direct-benefit angle.
- It helps you find shorter phrasings of something you’ve already written.
What AI doesn’t do well:
- It doesn’t know your readers the way you do.
- It tends toward generic, slightly polished-sounding lines that could belong to any brand.
- It sometimes over-promises, because it leans toward punchy, so you have to check whether the email actually delivers.
- It can’t tell you what will perform. Only your own list can do that over time.
That last point is the honest limit of everything I’m sharing. I’m early in building my list, so I can’t give you data on what wins. What I can give you is a process that avoids the worst mistakes and gets you to a decent subject line quickly.
How I Prompt AI for Email Subject Lines
The quality of what comes back depends almost entirely on what you give it. “Write me a subject line for my email” gets you something bland and generic. A good prompt has three ingredients.
Ingredient 1: the actual email content or purpose
Paste in the email itself, or at least a clear summary: what it’s about, what the reader will learn, and what you want them to do. The AI can only match the subject line to the email if it can see the email. This is the single biggest improvement you can make.
Ingredient 2: your brand voice
Tell it how you sound. For me, that’s honest, practical, no hype. If you leave this out, you’ll get the default energetic, exclamation-mark style that doesn’t sound like a real person. Even one line of voice instruction changes the output.
Ingredient 3: a request for multiple options
Never ask for one line. Ask for eight to ten in different styles. When you have options, you’re curating rather than hoping the first try is good. You’ll almost always find a better one by line five or six than at line one.
A prompt template you can copy
Here’s the structure I use. Swap in your own details:
Here is the email I’m about to send: [paste the full email or a clear summary].
My audience is [describe them, e.g. beginner bloggers building their first email list].
My brand voice is [e.g. honest, practical, plain English, no hype, no clickbait].
Give me 10 subject line options in different styles: some direct and clear, some with light curiosity. Keep them under 10 words where you can. Don’t use all caps, exclamation marks, or words that sound like spam. Make sure each one honestly reflects what’s in the email.
That last sentence matters. Telling the AI to stay honest to the content cuts down on the over-promising problem.
If you want a follow-up, ask it to give you five more in a different style, such as shorter, more personal, or more benefit-focused. Treat it like a conversation, not a vending machine.
My Actual Process, Step by Step
This is what I do each time I send an email.
Step 1: Write the email first. I never write the subject line before the email exists. Once I know what the email says, the subject line has something real to point at.
Step 2: Run the prompt. I give the AI the email content, my audience, my voice, and ask for multiple options.
Step 3: Read all the options, not just the first. I scan the full list and mark the two or three that feel closest to the email and closest to how I’d actually talk.
Step 4: Pick, tweak, or combine. This is the most important step. I rarely use a line exactly as given. Sometimes I take the structure from one and the wording from another. Sometimes I cut a word to make it shorter. Sometimes I rewrite one so it sounds like me.
Step 5: Do the honesty check. I ask myself: does this subject line match the email, and would I be comfortable if the reader compared the two? If not, I revise.
Step 6: Read it as a skimming stranger. I imagine seeing it in a crowded inbox, in half a second. Is it clear? Would I open it?
The whole thing takes a few minutes. Compared with the freeze I used to get, that’s a big improvement.
Why I Pick and Personalize Instead of Sending the First Suggestion
I’ll be blunt about this because it matters. Sending the first thing AI gives you is a bad habit, and here’s why.
First, AI-generated lines tend to share a recognizable sameness. As more people use the same tools with lazy prompts, inboxes fill with similar-sounding lines. Your edge as a small blogger is that you sound like a real person. Personalizing the line keeps that.
Second, only you know whether the line is true. The AI hasn’t felt the email the way you have. You’re the one who knows if the promise is honest.
Third, your list is your own relationship with your readers. You’re building something over months and years. A throwaway subject line sent on autopilot treats that relationship casually.
Think of AI as a drafting partner. It gives you raw material, and you make the decision.
A Quick Example of Pick, Tweak, Combine
Say my email is about why I run my opt-in popups through a separate tool instead of my main funnel platform. The AI gives me a list that includes things like:
- “Why my popups don’t live where my funnel does”
- “A small tool choice that fixed my opt-in popups”
- “The WordPress popup problem I had to work around”
None of these is perfect. The first is clear but a bit abstract. The third names a specific problem, which is good, but “had to work around” sounds a little negative. So I might combine the clarity of the first with the specific problem in the third, and land on something like “The popup problem I had to work around.” “ Shorter, honest, and it sounds like me.
That’s the move. The AI didn’t write my final line, but it got me to it much faster than I’d have gotten alone.
Common Mistakes I'd Steer You Away From
A few things I’d skip, based on the principles above:
- Writing the subject line first. You don’t know what the email promises until it’s written.
- Asking for just one line. You lose the benefit of choice.
- Skipping the voice instruction. Everything comes back generic.
- Chasing a “perfect” line. A clear, honest subject line sent on time beats a perfect one sent never.
- Copying other people’s clickbait style. It might work for large lists with different audiences, but for a new blogger, trust matters more than a short-term spike.
- Treating one email’s result as a verdict. With a small list, a few opens either way tells you almost nothing. Look at patterns over time.
A Note on the Tools Around It
AI helps with the writing, but your email platform is the thing that actually sends it. I use Systeme.io for most of my funnel, and I run my opt-in pop-ups through Brevo because Systeme.io’s pop-ups don’t integrate well with WordPress. That’s just what works for my setup, and it isn’t a recommendation that you copy it.
If you’re still choosing a platform, I’ve compared a few in other posts. My ConvertKit vs MailerLite comparison covers two popular beginner options, and MailerLite vs Systeme.io looks at a different pairing. Pick the tool that fits your budget and technical comfort. Subject lines work the same way in all of them.
And because an email list only matters if people find you in the first place, I’d also point you to my guide on how to rank blog posts on Google faster. Search traffic and your list grow together, and neither happens overnight.
The same pick-and-personalize approach also works for short copy elsewhere. I’ll cover that in my post on AI for Pinterest descriptions.
Where I Honestly Am Right Now
My blog is ten months old, and my list is small. I don’t have impressive open rates to show you, and I’m not going to invent any. What I’m doing is staying consistent, sending emails, using a process I trust, and letting both Google and my list grow at their own pace.
I think that’s more useful to you than a polished success story. If you’re a beginner staring at your first subject line, you don’t need to see someone at the finish line. You need to see someone a few steps ahead who’s doing the unglamorous work and can tell you what’s actually helping.
Frequently Asked Questions
Can AI really write good email subject lines?
It can write useful first drafts, especially when you give it the real email content and clear instructions. It isn’t reliably great on its own. The best results come from generating several options and then choosing and editing the one that’s honest and sounds like you.
How many subject line options should I ask AI for?
I ask for around ten. That’s enough variety to find a few strong candidates without being overwhelming. If none feel right, ask for another batch in a different style.
Should I send the first subject line the AI suggests?
No. The first suggestion is usually the most generic. Read the full list, then pick, tweak, or combine. You also need to check that the line truthfully matches your email.
What makes a subject line count as clickbait?
A subject line is clickbait when it promises something the email doesn’t deliver. Curiosity is fine, but it needs a real payoff inside. A good test: if the reader compared the subject line to the email, would they feel misled?
Do I need to mention my brand voice to the AI?
Yes, it makes a noticeable difference. Without it, you tend to get a generic, overly excited tone. Even one sentence like “honest, practical, no hype” steers the results toward something that sounds like you.
How long should an email subject line be?
There’s no perfect length, but shorter usually travels better on phones, where long lines get cut off. Put the most important words first and try to stay under about ten words.
Will using AI for email subject lines guarantee a higher open rate?
No, and anyone who promises that is overselling. Open rates depend on your audience, your sender reputation, your timing, and your content. AI helps you write clearer options faster. It can’t guarantee a result.
Conclusion
Writing a subject line feels hard because it’s the smallest part of your email carrying the biggest job: convincing a busy person to open it. Using AI for email subject lines takes the pressure off by replacing the blank field with a list of options to choose from.
The process I rely on is simple. Write the email first. Give the AI the actual content, your brand voice, and ask for multiple options, not just one. Then read the whole list, pick or combine what works, tweak it so it sounds like you, and do an honesty check to make sure the line matches what’s inside. The principles underneath are just as simple: clarity beats cleverness, curiosity needs a payoff, and the subject line has to keep the promise it makes.
I’m still early in building my own list, and I won’t pretend otherwise. That’s exactly why I trust this approach: it’s built on honesty and consistency rather than tricks. Your first few emails won’t be perfect, and that’s fine. Start with a clear process, send the email, and learn from your own list over time. Pick one email you’re working on today, run the prompt above, and see how much easier that blank box becomes.
Quick disclosure: I’m an affiliate for Systeme.io, one of the tools I mention below. If you ever sign up through one of my links, I may earn a commission at no extra cost to you. I only mention tools I actually use.
