AI Knows How to Make the Best Email Subject Lines

The ACL 2019 study introduced the first email subject generation dataset. It analyzed hundreds of thousands of emails — subjects, bodies, open rates — to teach machines what makes people click.

What they came up with
This is a real challenge. Researchers set out to automatically generate an email subject line (SLG) based on the email’s body text. In some ways, it resembles creating news headlines, but email subject lines are much shorter, which means the content must be summarized with a very high compression rate. And until 2019, no one had attempted this!

Technically, the generation process distills the email content down to its essentials. Initially, the model extracts key information from the email body, including entities, dates, and times. After that, an abstraction module rewrites several selected sentences into a concise subject line, preserving the core information. Pretty impressive, right?

The most interesting part is that, according to research data, machine-generated subject lines by ACL outperform competitive baseline metrics and approach a quality comparable to human-written ones.

Core results

• The model learns linguistic and emotional patterns.

• Optimal length: 6–10 words.

• Emotional verbs increase open rates by 15–18%.

• Numbers still help, but less than before.

• Overused phrases like “exclusive” or “personal” backfire.

Example
Instead of guessing, AI suggests:

• “Your favorites are back in stock.”

• “What everyone’s buying this week.”

• “You left something behind.”

Marketers pick, test, and refine. The system evolves with brand tone and performance data. After all, AI doesn’t replace creativity — it scales it. Marketers can now analyze thousands of campaigns and generate copy that actually learns from success.

Takeaway for marketing

• Test subject lines systematically. Let an AI model or A/B testing planner evaluate your variations.

• Monitor word fatigue. Anything that sounds overly “marketing” quickly loses effectiveness.

• Don’t fear length. Longer subject lines get read if they are genuinely engaging.

• Create a top-performers archive. Collect 50 subject lines with the highest open rates to use as a training dataset.