What makes a good email subject line?
A good email subject line names what the reader gets from the email, in words the reader would use. "Fill last-minute cancellations from your waitlist" tells a clinic manager why the email is worth a minute. "Product update: October" tells them only that you have news. Most people decide whether to open in the second or two they spend scanning the inbox, so the subject line has to carry the email's most useful idea on its own.
The preview text matters almost as much. It's the line most inboxes show beside or below the subject, and when it's left empty, the inbox fills it with the first words of the email, which are often "View this email in your browser." A subject line and preview text that work together read like a headline and its first sentence.
Writing a strong subject line is only half the job. The other half is learning which kinds of subject lines your own audience responds to, and that takes testing. A simple test sends two versions of the subject line to a small share of your list, waits a few hours, and sends the better one to everyone else. Most email platforms can run that test. The hard part is writing two versions that differ in a way worth learning from, then reading the results honestly and remembering the lesson for the next send.
An AI agent is well suited to this work because it can do all three parts with your history in front of it. It can read every subject line you've sent and what each one earned, write new versions from what it knows about your readers, and keep a record of what each test taught you.
Kite is an AI marketer. It researches your business, builds a growth strategy, and ships the work that brings you new customers. Writing and testing subject lines is part of how it runs your email campaigns. Here's how it works.
How Kite writes and tests your email subject lines
Kite starts from what it already knows. During onboarding, it researched your company and built a shared knowledge base with your positioning, products, customers, and brand voice. When you connect Mailchimp or Brevo, Kite can also read your past campaigns, including the subject line each one used and the opens and clicks it earned.
When Kite writes an email campaign, it settles the email's plan before it writes the email. The plan names the one job the email has, the proof it rests on, the call to action, and the subject line and preview text. Kite returns the plan beside the draft, so you can judge the subject line against what the email is meant to do.
When you want a test, Kite writes two subject lines that differ in one idea and tells you what the test is meant to show. After the send, it reads the results for each version and records what it learned. The next campaign starts from that lesson instead of a blank page.
- Start from what your past subject lines earned
- Write subject lines that name what the reader gets
- How do I A/B test email subject lines?
- Read the results and carry the lesson forward
Start from what your past subject lines earned
The fastest way to improve your subject lines is to look at the ones you've already sent. Kite reads your past campaigns in Mailchimp or Brevo and puts each subject line next to its opens and clicks. That usually shows a pattern a single send can't. Maybe the emails that named a specific feature earned twice the clicks of the ones that named the month, or maybe every subject line written as a question underperformed.
Kite also notices when your subject lines have started to sound alike. If the last six all began with "New:", readers have learned to skim past them. Kite treats the subject lines your readers have already seen as a list to avoid repeating, so each new one gives them a fresh reason to open.
Write subject lines that name what the reader gets
Kite writes each subject line from the email's one job and the readers it's going to. It draws on the positioning, customer research, and brand voice in your knowledge base, so the subject line uses the words your customers use for their own problems.
Here's an example. Say you sell scheduling software to dental clinics, and this month's email announces a waitlist feature. A typical subject line would be "October product update." Kite would write something closer to this:
- Subject line: Fill last-minute cancellations from your waitlist
- Preview text: When a patient cancels, the next person on your waitlist gets a text and can book the slot.
The subject line names what the reader gets. The preview text explains how, in one plain sentence, without repeating the subject. Kite writes in your brand voice, so a company that writes casually gets a casual subject line, and a company that writes everything in lowercase gets lowercase.
Kite keeps the claims in a subject line true to the email. If the email is a product announcement, the subject line doesn't promise a discount. If the email offers a discount, the subject line names the real reason for it, such as the end of a season, rather than inventing a deadline.
Related Pages
- Email subject lines that get opened: real examples and how to test yours Coming soon Real subject lines grouped by the job each email had, with what testing showed about them.
How do I A/B test email subject lines?
You A/B test a subject line by sending two versions to a small share of your list, picking the version that performed better, and sending it to everyone else. The test only teaches you something if the two versions differ in one idea. If version A is short and specific and version B is long and playful, you'll know which one won but not why.
Kite writes the two versions around a single question worth answering. For the dental clinic email above, the test might compare a benefit with a question:
- Version A: Fill last-minute cancellations from your waitlist
- Version B: What happens to the 2 p.m. slot that just cancelled?
Both versions describe the same email, and both are honest about it. The only thing that changes is whether the subject line states the benefit or asks the reader to picture the problem. Whichever one wins, you've learned something about your readers that applies to the next email.
In Mailchimp, Kite can set up the test for you. Mailchimp sends the two versions to part of your audience, waits, and sends the winning version to the rest. Kite runs Mailchimp's send checklist first, sends a test email to your inbox so you can see both subject lines the way your readers will, and schedules the campaign once you approve it.
In Brevo, Kite writes the subject line and content into the campaign you've started there. Brevo's own campaign editor has an A/B test option, so you can paste in Kite's second version and run the test from Brevo. Kite reads the campaign's results afterward.
Read the results and carry the lesson forward
Kite judges a subject-line test by clicks as well as opens. Open rates have become less reliable since Apple Mail began loading emails in the background for privacy, which counts many Apple Mail readers as having opened an email whether they did or not. A subject line that earns more opens but fewer clicks may have promised something the email didn't deliver. Clicks show whether the subject line brought in the people who actually wanted what was inside.
After the send, Kite reads the report for each version. In Mailchimp, that includes deliveries, opens, clicks, and unsubscribes, plus which links people clicked. In Brevo, it includes deliveries, opens, clicks, bounces, and unsubscribes for the campaign. Kite tells you which version won, by how much, and whether the difference is large enough to trust. On a list of a few hundred people, a gap of a handful of opens is usually noise, so Kite will suggest running the same kind of test again on the next send before treating the result as a rule.
A campaign that's already gone out can still earn more opens. In Mailchimp, Kite can resend the campaign to the subscribers who didn't open it, usually with a new subject line written from what the first send showed.
When a campaign is finished, Kite records what worked in your knowledge base: the subject lines, the angles, and the results they earned. The next time Kite writes a subject line for the same readers, it starts from those lessons. Over a few months, that record becomes a clear picture of what makes your audience open.
What you get
A subject line and preview text for every campaign
Each one names what the reader gets, matches your brand voice, and differs from what the same readers have seen from you before.
The email's plan beside the draft
The email's one job, its proof, and its call to action, so you can judge whether the subject line fits the email.
Two test versions with a reason
Subject lines that differ in one idea, with a plain statement of what the test is meant to learn.
A results summary
Opens and clicks for each version, which one won, and whether the result is strong enough to act on.
A growing record of what works
The lessons from each test, saved in your knowledge base and used in every subject line Kite writes after that.
How to get started
You can work with Kite in your browser or in Slack. Start by telling Kite what you want in plain language. You might write, "Our newsletter open rates are low. Look at our last ten campaigns and tell me what's going wrong with the subject lines," or "Write this month's product email and set up a subject-line test in Mailchimp."
Kite comes back with what it found and a plan. For a new campaign, that means the draft email, its plan, and the subject line and preview text, or two versions if you asked for a test. Edit anything you like. By default, Kite asks for your approval before any email goes to your audience, and in Mailchimp, it can send a test to your inbox first.
To have Kite read your past results and run tests, connect the email platform where your subscribers live. Mailchimp connects through OAuth, so you sign in to your account and approve the connection. Brevo connects with an API key you create in your Brevo account settings. You can also ask Kite for subject lines without connecting anything, and it delivers them ready to paste into the tool you use.
Integrations used to write and test email subject lines
Kite writes subject lines from what it knows about your business, so you can get your first set without connecting anything. Connecting your email platform lets Kite read what your past subject lines earned, put new ones into your campaigns, and measure the results.
Common questions
How long should an email subject line be?
Keep it short enough that the important words show on a phone. Many phone inboxes show only the first 30 to 40 characters of a subject line, so put the part that matters first. "Fill last-minute cancellations from your waitlist" still makes sense if it's cut off after "cancellations." Length matters less than clarity, though. A clear subject line of eight words beats a vague one of three.
Should I put the reader's first name in the subject line?
It depends on the email. A first name can help a one-to-one email feel personal, but on a newsletter it's become so common that many readers no longer notice it. It's a good thing to test rather than assume. Kite writes campaign subject lines about the readers' shared situation rather than facts about an individual, and it can test a version with a first name against one without.
How is Kite different from an AI subject line generator?
A subject line generator writes a list of options from a topic you type in. It doesn't know your readers, what you sent them last month, or which subject lines earned clicks. Kite writes subject lines from your positioning, your brand voice, and the results of your past campaigns. It also sets up the test, reads the results, and remembers the lesson for the next send.
Can I test subject lines if my list is small?
Yes, but read the results carefully. On a list of a few hundred people, the difference between two versions is often too small to trust from a single send. A better approach for a small list is to test the same kind of change, such as a question against a statement, across several campaigns. Kite keeps track of those results in your knowledge base, so a pattern can emerge even when no single test is decisive.
Is there a free way to write and test subject lines with AI?
Kite has a free plan that includes usage, so you can ask it to review your past subject lines or write new ones without paying. Testing runs in your email platform under your own plan with that platform. If you need more, Kite's Growth plan costs $100 a month and includes $100 in AI credits.