Introducing AI-Powered Subject Line Alternatives: On-Device AI Meets Email Marketing

How we started

SendCheckIt started as a full featured automated QA SaaS product for email marketers (which people didn't really care about) and to help promote it, I'd built an email subject line tester as a fun little tool (which people loved), so I ditched the SaaS idea and focused on the subject line tester as a free standalone tool.

The tester takes a subject line and scores it on a number of different factors, including readability, spam triggers, length, sentiment, and more.

To build it, I hired a bunch of people off of Fiverr to subscribe to thousands of email newsletters whose contents I did a statistical analysis on to figure out how to reverse engineer the best subject lines.

This worked really well and since then we've tested literally millions of subject lines for a 100,000+ email marketers.

The limits of usefulness

But, and this is kind of weird, the subject line analyzer is much better at telling you what not to do than it is at telling you what to do.

It will prevent you from making dumb mistakes (like putting "FREE!!!!" in your subject line), but I always felt kind of lame that it couldn't take the next step and actually riff off of the user-entered subject line and come up with alternatives that might be better.

Just use AI dummy

Obviously, this is something AI is good at in the abstract. From emailing with folks using the subject line analyzer, I know that people are bouncing between ChatGPT and SendCheckIt.

They'll put a subject line into ChatGPT and ask it to "generate 5 alternative subject lines for this email" and it will cough up a list of 5 alternatives...but they're based on your prompt and vibes and will often contain spam trigger words, be too long, or just not be very good.

The system is popular, free and gets a wild amount of abuse making me very reluctant to add potentially expensive AI features.

People will then copy and paste different subject lines into SendCheckIt to check the scores until they find something that fits their email, has a high score and isn't spammy.

So, while I've technically had the ability to add this to the subject line analyzer for a while, there's been three major issues holding me back:

1. Cost
2. Cost
3. Cost

The system is popular, free and gets a wild amount of abuse making me very reluctant to add server based AI features as even low cost AI models expose me to a potentially massive AI bill.

A brief introduction to AI models

If you're reading this, it's likely you're a marketer and not a developer. And you may reasonably think of different types of AI models in the same way you think of databases, smtp servers or DNS resolvers: aka you don't.

But, all that's important to know is that there are "big" AI models that typically run on a server (like when you go to ChatGPT.com or Google Gemini) and there are "small" (or local) AI models that typically run on your device.

Here are the different types of Google's Gemini AI models:

• Gemini Ultra - largest, most capable (cloud only)
• Gemini Pro - mid-tier (cloud API)
• Gemini Flash - fast, efficient (cloud API)
• Gemini Nano - smallest, runs locally on-device

And this last one: Gemini Nano is the answer to our prayers.

It's an AI model that runs entirely within Chrome, makes no network calls, is free to use and gives reasonable results.

How does this look in practice?

When you test a subject line now, you'll see a new section: AI-Powered Alternative Subject Lines. Enter your subject, and within seconds you'll get five alternatives—each automatically scored using the same algorithm that graded your original.

The alternatives aren't random suggestions. They're contextually aware of your specific issues:

• Subject too long? You'll get concise alternatives under 40 characters
• Spam words detected? The AI avoids those triggers entirely
• No personalization? One alternative will include {first_name} merge tags
• Not engaging enough? You'll get curiosity-driven and question-based options

Each suggestion uses a different approach—curiosity, questions, benefits, brevity, or personalization—so you're not just getting variations of the same idea.

What about non Chrome browsers?

Fortunately, most of our users are on desktops and using Chrome as this works only in Chrome.

What about other browsers?

Firefox, older Chrome versions, mobile users, Safari and other browsers will unfortunately not see the AI-Powered Alternative Subject Lines section, but we put it in place in such a way that it just cleanly won't show (no errors, no broken UI).

As these add their own AI models, we can add support for them in the future.

What now?

Give it a try: Email Subject Line Tester and let me know what you think -> mike@sendcheckit.com

Thanks,

Mike