I Built an AI Email Editor — Here's What I Learned About What Makes Emails Convert

When I started building Punchline, I thought the hard part would be the AI. Getting Claude to generate decent email copy, designing a visual editor, wiring up the Brevo API — that's the technical challenge.

But the real education came from somewhere I didn't expect: the data.

Punchline connects to your Brevo account and reads your campaign history before generating anything. It looks at what you've sent, what got opened, what got clicked, and what got ignored. After months of building this system and watching it analyze thousands of campaigns across dozens of accounts, patterns started to emerge.

Here's what I learned about what actually makes emails convert — and what doesn't matter nearly as much as people think.

The subject line is not the hero

Everyone obsesses over subject lines. And yes, they matter — they determine whether someone opens your email. But here's the thing: a great subject line with a bad email is worse than a good subject line with a great email.

Why? Because the person who opens your email and finds nothing compelling inside won't click. And they'll be less likely to open your next one. You've spent your credibility on an empty promise.

The campaigns with the highest click-through rates in our data weren't the ones with the cleverest subject lines. They were the ones where the subject line accurately promised something, and the email immediately delivered on that promise.

Lead with the value, not the greeting

The most common pattern I see in underperforming emails: they start with "Hi [Name]" followed by two paragraphs of context before getting to the point.

Your reader decided to open this email based on your subject line. They're looking for the thing you promised. Every sentence between the open and the payoff is a chance for them to leave.

The best-performing emails put the value proposition in the first line. If you're announcing a sale, the discount is right there. If you're launching something, the product is front and center. The greeting can come after — or not at all.

This was one of the first rules I baked into Punchline's generation. The AI never opens with a greeting. It leads with whatever the email is actually about.

One email, one job

Another pattern that showed up clearly in the data: emails that try to do multiple things convert worse than emails that do one thing well.

"Check out our new product AND here's a discount code AND also read our latest blog post AND follow us on Instagram" — that's four calls to action competing for attention. The reader doesn't know what you want them to do, so they do nothing.

The highest-performing campaigns have one clear goal. Everything in the email — the headline, the body, the images, the button — points toward a single action.

Design matters less than you think (but more than zero)

I expected beautiful emails to perform better. They don't — at least not reliably. What I found is that consistent branding performs well. When your email looks like it came from you — your colors, your logo, your voice — it feels familiar and trustworthy.

But pixel-perfect design? Elaborate layouts? Custom illustrations? The data doesn't support them as conversion drivers. Clean, readable, on-brand beats fancy every time.

That's why Punchline extracts your brand elements automatically — colors, fonts, logo — and applies them consistently. It doesn't try to create Instagram-worthy designs. It creates emails that look like they came from your brand and are easy to read.

Urgency works, but only when it's real

Urgency is the most effective conversion lever in our data. Emails with genuine deadlines, limited stock, or time-sensitive offers consistently outperform evergreen content.

The key word is genuine. "Last chance!" in every email trains your audience to ignore it. But "This offer ends Friday" when the offer actually ends Friday? That works.

Punchline asks for deadlines when you're creating a promotional email. If you give it one, it weaves it naturally into the copy and can add an urgency bar to the design. If you don't have one, it doesn't fake it.

What I'd tell myself before I started

If I could go back to before I built Punchline, here's what I'd tell myself about email marketing:

1. Don't overthink the subject line. Make it accurate and specific. "30% off running shoes this weekend" beats "You won't believe what's inside" every time.
2. Get to the point immediately. Your first sentence should deliver on whatever the subject line promised.
3. Pick one goal per email. If you have multiple things to say, send multiple emails.
4. Be consistent, not creative. Your brand identity matters more than clever design.
5. Use urgency honestly. Real deadlines drive action. Fake ones erode trust.

These aren't revolutionary insights. But they're the patterns that actually show up in the data, across industries and audience sizes. And they're the principles that Punchline uses to generate emails that work.

If you're a Brevo user and you want to see what this looks like in practice, try Punchline for free. Generate an email and see if the patterns match what works for your audience.