Can Artificial Intelligence Write Better Email Subject Lines Than Humans?

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Paul Roetzer
Paul Roetzer


Can AI write a better subject line than an actual person? Probably. Brands like JOANN have had a 10% increase in email open rates, all thanks to AI.

ai email subject line represented by a letter

Unless you’re an expert email marketer, coming up with brilliant subject lines every time can be tough. This is especially true if you’re sending personalized emails multiple times per week.

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Plus, you may need to write several subject lines and run split tests to find the best. This is time-consuming!

This post will explore how AI works with email subject lines and why you should get inspiration from AI when writing email subject lines. You’ll also learn about some of the best AI email subject line generators.

How AI Works With Email Subject Lines

Many AI technologies are still in their infancy. They look good on paper but haven’t reached their potential. That said, there are other AI content generators that have advanced rapidly to become game-changers.

It’s this second group that is driving the AI explosion we’re seeing today. Two technologies, in particular, have driven AI progress:

  1. Natural language processing (NLP)
  2. Natural language generation (NLG)

NLP is when a machine “reads” text. It turns that text into code a machine system can then interpret. NLG is when a machine uses that code to generate its own words.

These technologies are found in AI-powered marketing tools like Acrolinx, which uses NLP to assess your content and recommend improvements. One other major use case enabled by NLP and NLG is, you guessed it, writing email subject lines.

Besides saving time, another benefit of AI tools is that developers train them on email engagement using a large amount of data.

These data are based on email best practices, enabling the AI tools to create bias-free subject lines that could cause high open rates.

As Parry Malm puts it, “When humans make choices, they inject their own experience, thoughts and beliefs.” This can cause bias and make us opt for subject lines that may not be the best.

But AI is free of bias, and it’s trained using clean data. “AI output is based on pattern matching, which is invisible to the human eye and devoid of cognitive biases that lead to poor decisions,” Parry adds.

Now, let’s explore some of the best AI tools for generating email subject lines.

4 AI Tools for Generating Email Subject Lines

I tried most of the AI email subject line generators and barely found those that made the cut. Some of the output didn’t follow best practices, as the subject lines were long.

Below are four of what I consider the best AI email subject line generators in 2023.

1. HubSpot AI Email Writer

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HubSpot's AI Email Writer enables you to automate email copywriting for sales outreach, email marketing, and more. It is versatile, allowing you to generate email copy for diverse audience segments by prompting it to write in different tones.

You can select from email marketing templates and use the slash and highlight commands to come up with emails that suit your needs. 

Furthermore, HubSpot's AI Email Writer conveniently integrates with the HubSpot CRM to to automatically log conversations and contacts.

Pricing: Free.

2. Persado

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Best for: Enterprise companies

Persado is an AI platform that allows users to personalize communications at scale and inspire an audience to engage and act.

The brand has $66 million in funding and offers AI email subject line writing, in addition to automatic social media and language generation for enterprise marketing.

One reviewer, who is a senior email marketing and analytics specialist, says:

“We’ve seen a strong increase in open rates with Persado, and we’ve been able to learn about our audience. Persado categorizes subject lines by emotion, provides insights on the language that motivates your audience, and removes the pain of writing high-volume subject lines.”

Pricing: Pricing is available upon request.

3. Phrasee

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Best for: Mid-size and enterprise companies

Phrasee uses AI to create email subject lines, write email copy, and generate email CTAs. You feed it email data, and it analyzes the data to learn what works.

It generates optimized subject lines that can perform better than human-generated ones. And it uses what it learns from your emails to stay consistent with your brand.

In one case study, Phrasee helped Virgin Holidays increase their open rates by 2%. That may not sound like much, but it was worth millions of new revenue. The tool also streamlined a process that used to take weeks — today, it takes seconds.

Pricing: Phrasee’s pricing starts at $500 per month, billed annually. To access enterprise features, you’d have to contact Phrasee’s sales team for a quote.

4. ChatGPT

Best For: Individuals and companies seeking a free AI tool

There’s a good chance you’ve heard about ChatGPT. ChatGPT is a free AI tool that provides the best responses based on the quality of your prompts.

When using ChatGPT, it’s a good idea to not just tell it to create a subject line. Include the email you want to send and ask it to create subject lines for that email.

Here are two examples of prompts you can use to get subject lines:

  • “Create 10 subject lines based on the email below. Each subject line should include a potential percentage of its chance of getting opened. Here’s the email…”
  • “Write 10 email subject lines for the email below. Each subject line should end in a question and have one emoji that shows curiosity.”
  • “Write 10 email subject lines for the email below. Each subject line should contain seven words or fewer and end with a question mark.”

Pro Tip: To get the most from ChatGPT, you have to be ultra-specific with your prompts.

Leveraging AI Email Subject Line Generators

One of the beautiful elements of AI is that it operates with little human involvement. At HubSpot, we see value in this.

That’s why we’re developing HubSpot AI — a content assistant and ChatSpot for generating email subject lines and even creating emails from scratch.

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It’s hoped that tools like this will ensure we don’t stay married to our creative ideas.

See, humans aren't that great at writing and split-testing emails, creating social media content, writing headlines, and picking the best subject lines based on the data.

Heck, we sometimes do these wrong. Here, AI is better equipped than us for this task. It excels at analyzing vast volumes of data and extracts insights from that data.

For instance, Phrasee routinely uses 100,000+ emails as a dataset. That’s a lot!

So, often, AI can write email subject lines better (and faster) than humans. That’s good news because it improves your marketing.

Instead of drowning in data and having little time or budget to maximize performance, marketers can use AI to automate and augment their processes. They both improve performance and free you up to do higher-value tasks.

In a lot of cases, this is a win-win scenario. Marketers get better results and do more of what benefits their brand most (which also is the most interesting stuff anyway).

This is why marketers need to experiment with AI. Understand the tools out there and know what they can do for you.

AI in marketing right now has a “first-mover advantage.” Let’s say I start using a tool like Phrasee to write my email subject lines.

I improve my performance almost immediately. I reinvest hours each week into building better campaigns in every area of our marketing.

And, I collect more and more data on what subject lines work best, improving performance even more. It’s a virtuous cycle. A flywheel that spins faster and faster the longer you stick with it.

In other words, the advantages of using AI correctly compound over time. It’s hard to catch competitors with a sizable head start. Someday, that may change.

But right now, marketing AI is skyrocketing. Brands using it possess an outsized advantage over those that don’t.

The Possibilities of AI

No matter what type of marketing you do, one thing is clear: You need to explore AI's possibilities. These tools can help you write the best email headline and improve the copy within.

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