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AI subject line tools compared: which ones actually improve open rates?

metrics_mikeData Analyst

There are dozens of AI subject line tools now. We ran a structured test over 3 months, sending 60 A/B tests (AI-generated vs. human-written) across our 80k subscriber list.

Tools tested

  1. Brew AI — Built into the platform, uses your historical data
  2. Phrasee — Enterprise-focused NLP tool
  3. SubjectLine.com — Free scoring tool
  4. ChatGPT — Custom prompts for subject lines
  5. Copy.ai — Marketing copy generator

Results

Brew AI won 62% of A/B tests against human-written subjects. It has the advantage of being trained on your own send data, so suggestions are contextually relevant. Phrasee was close at 58% but costs 10x more.

ChatGPT performed surprisingly well at 54% when given good prompts and examples. The free tools were essentially random — no better than human.

Key insight: AI subject lines work best when they have access to your historical engagement data. Generic AI tools produce generic results.

#ai#subject-lines#ab-testing#brew
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5 Comments

growth_graceGrowth Marketer

We have been using Brew's AI subject lines for 2 months. Can confirm the improvement is real. Our open rates are up about 10% overall.

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copywriter_chrisEmail Copywriter

Interesting that Brew AI outperformed Phrasee. Is this because Brew has access to your send data directly?

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metrics_mikeData Analyst

Yes, exactly. Brew's AI trains on your historical engagement data, so suggestions are contextually relevant to your audience. Generic AI tools lack that context.

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newsletter_nerdNewsletter Creator

The finding that AI needs your data to be effective is important. Off-the-shelf subject line tools that do not integrate with your ESP are not much better than random.

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data_diego

The key insight is that AI needs YOUR data. Generic subject line tools are basically random. Platform-integrated AI like Brew's learns from your specific audience patterns.

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