AI Email Subject Lines: 4 Prompts and a Simple Test Plan
Quick answer: Give the AI specific context: who reads the email, what it offers and one concrete detail. Ask for 10 to 20 options in different styles, cut anything spammy or vague, then A/B test the best two on a small part of your list and send the winner.↗ Share on X
To use AI for email subject lines that get opened, give the tool specific context (who the reader is, what the email offers, and one concrete detail), ask for 10 to 20 options in different styles, cut anything that sounds like spam or hype, and then test the best two against each other on a small part of your list. AI is great at producing many options fast. It is bad at knowing your audience. Your job is to feed it the right details and pick the winner with real data.
This article gives you a step-by-step process, copy-ready prompts, a checklist to avoid spam filters, and a simple way to test what works.
Why do most AI subject lines fall flat?
Keep AI Writing On-Brand: A 5-Step Voice Setup That Works →
Stack AI Writing Tools for SEO: A 7-Step Workflow That Works →
AI Product Descriptions That Sell: A 7-Step Process →If you type "write a subject line for my newsletter," you get something like "Unlock Exclusive Insights Inside!" It sounds like every other email in the inbox. Readers skip it, and some spam filters may treat that kind of wording with suspicion.
The problem is not the AI. The problem is the prompt. AI fills gaps with the most common phrases it has seen. If you give it nothing specific, you get the most average result possible.
Three things usually go wrong:
1. No reader in mind. The AI does not know if you write to busy moms, plumbers, or software buyers.
2. No real detail. "Big sale" is vague. "30% off winter boots until Sunday" is specific.
3. One option only. The first idea is rarely the best one.
Smart software picks in your inbox
Step 1: What information should you give the AI?
Before you open any AI tool, write down these five things. It takes about two minutes and makes the biggest difference.
| Detail | Example |
|---|---|
| Who reads it | Small bakery owners on our customer list |
| What the email is about | New guide on pricing cakes for profit |
| The one useful detail | Includes a free price calculator sheet |
| What you want them to do | Open and download the sheet |
| Your brand voice | Friendly, plain, a bit funny, no hype |
Also note any words to avoid. If your audience hates "hacks" or your company never uses emojis, say so.
Step 2: Which prompts actually work?
Refresh Old Blog Posts With AI: 4 Prompts That Lift Clicks →
AI Content Briefs for SEO: A 7-Step Process That Works →
How to Edit AI Drafts So Google Sees Them as Helpful →Copy one of these and fill in the brackets.
Prompt for variety:
Write 15 email subject lines for [audience]. The email is about [topic]. The main benefit is [specific detail]. Voice: [voice]. Give 3 of each style: a question, a number, a curiosity gap, a direct benefit, and a personal tone. Keep each under 50 characters. Do not use all caps, more than one exclamation mark, or words like "free money," "act now," or "100% free."
Prompt for a preview text pair:
For each subject line above, write a matching preview text (the gray line shown after the subject in most inboxes) of 40 to 90 characters that adds new information instead of repeating the subject.
Prompt to sharpen your favorite:
Here is my best subject line: "[line]". Write 8 variations that keep the same idea but make it more specific, shorter, or more surprising.
Prompt to make it sound human:
Rewrite these subject lines so they sound like a real person wrote them to one friend. Remove any marketing buzzwords.
Step 3: How do you pick the best options?
You will have 15 or more lines. Cut them down with this quick filter:
1. Would you open it? Be honest. If it bores you, it bores them.
2. Is it specific? Lines with a number, name, date, or clear result usually beat vague ones.
3. Does it match the email? A clever line that promises something the email does not deliver hurts trust. People stop opening future emails.
4. Is it short enough? Many phones cut subject lines after roughly 30 to 40 characters. Put the important words first.
5. Does it pass the spam check? See the next section.
Keep your top two or three. Those go to testing.
Step 4: How do you avoid spam filters?
Spam filters look at many things, including your sending reputation, your list quality, and how people engage with your emails. The subject line is only one part, but it is the part you control most easily. Avoid these common red flags:
- ALL CAPS WORDS
- Several exclamation marks or question marks in a row (!!!, ???)
- Money promises like "earn $$$" or "make money fast"
- Pressure words like "act now," "urgent," or "last chance" used every week
- Misleading starts like "RE:" or "FWD:" when it is not a reply
- Too many emojis
- Subject lines that have nothing to do with the email inside
A useful habit: ask the AI to review its own work. Try this prompt:
Review these subject lines and flag any that could look spammy, misleading, or overhyped. Explain why and suggest a cleaner version.
Then use your own judgment. The AI can miss things, especially rules specific to your industry.
Step 5: How do you test which subject line wins?
Most email tools (like the ones used for newsletters and online stores) have an A/B test feature. It sends version A to one small group and version B to another, then sends the winner to everyone else.
Here is a simple way to run it:
1. Pick only two lines that are clearly different, like a question vs. a number.
2. Send each to a slice of your list. Many senders use 10% to 20% for each version. With a very small list, send to half and half and just learn from it.
3. Wait a few hours before picking a winner. Many tools let you set this automatically.
4. Look at opens, but also clicks. Some email apps open messages automatically for privacy reasons, which can make open rates look higher than they are. Clicks show real interest.
5. Write down what won and why. After five or six tests, you will see patterns for your own audience.
| Test | Version A | Version B | What you learn |
|---|---|---|---|
| Style | "How to price a cake for profit" | "Are you losing money on every cake?" | Do your readers prefer how-to or a question? |
| Number | "Cake pricing tips inside" | "3 cake pricing mistakes to fix today" | Do numbers help? |
| Personal | "New pricing guide" | "I made this calculator for you" | Does a personal tone work? |
Which AI tools can you use?
Almost any general AI writing tool can write subject lines: chat assistants, the AI built into many email platforms, and writing apps with AI features. Many have free versions that are enough for this task. The tool matters less than the prompt and the testing.
A few practical notes:
- Built-in AI in your email platform is convenient because you write and test in one place.
- A general chat assistant gives you more control and lets you refine in a back-and-forth.
- Do not paste private customer data into any tool unless you know how that tool stores it. Use general descriptions of your audience instead.
Common mistakes to avoid
- Using the first result. Always ask for many options.
- Chasing opens with clickbait. It may work once. It trains people to ignore you.
- Changing the subject and the email at the same time. Then you cannot tell what made the difference.
- Forgetting the preview text. It is free extra space to convince the reader.
- Not testing at all. Your opinion is a guess. Your list's behavior is data.
Your next step
For your next email, take two minutes to fill in the five-detail table from Step 1. Paste it into the variety prompt above and ask for 15 subject lines. Cut them to two using the five-question filter, set up an A/B test in your email tool, and send. Write down the winner in a simple note or spreadsheet. Do this for your next five emails, and you will know more about what makes your readers open than any generic list of tips can tell you.
FAQ
Can AI write good email subject lines?
Yes, if you give it specific details about your audience and offer. AI is fast at producing many options, but you still need to choose and test them with your own list.
How long should an email subject line be?
Shorter is usually safer. Many phones cut subject lines after roughly 30 to 40 characters, so put the most important words at the start.
Do certain words send emails to spam?
Spam filters weigh many factors, including sender reputation and engagement. Still, all caps, lots of exclamation marks, money promises and misleading RE: tags can hurt, so avoid them.