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Three Rules Of Subject Line Testing with AI

“O Subject Lines, how you test us!” 

That is supposed to be the other way around, but subject line testing can feel like a chore. It tests our patience even more when the results don’t give us the lift we hoped for. That’s where adding some Artificial Intelligence (AI) like ChatGPT can help – if you ask nicely. 

ChatGPT has been in the news a lot lately. It is a large language model trained by OpenAI, which in simpler terms means it can write in a way that sounds like a real person. Here at Alchemy Worx, we did a ton of testing, asking ChatGPT all sorts of questions as nicely as possible. We found out that ChatGPT doesn’t have a favorite ice cream flavor, but does think email can be useful. Good enough for us!

We integrated ChatGPT into our well-tested Subject Line Pro platform and got to work. Subject Line Pro virtually tests subject lines and delivers accurate results within seconds. (Learn how you can get a Free month of Subject Line Pro below.) Now with ChatGPT added, you can test multiple subject lines quickly and easily before letting them out in the real world. It’s up to you to add the strategy, so we are sharing our three rules to get the most out of your subject line testing. 

Rule 1: A subject line must include the facts. What is the email about? “50% off Baked Starter Kit” or “Spring planting season starts today” are excellent starting points. A good subject line should be relevant to the content of your email. Don’t overthink it. Just write the facts that need to be included. 

Rule 2: A subject line should obey your testing goals. Think about testing tone or theme. Would your audience react better to FOMO or low-key tone? Should you explore a holiday theme or a seasonal approach? Should you be funny or serious, creative or straightforward? You can explore several options and then decide. This is where knowing your audience and what they expect from your brand voice will be important.

Rule 3: A subject line needs to protect its ability to perform. It’s a good idea to set rules around character counts or length, and any phrases or words to avoid. ChatGPT doesn’t realize that inboxes sometimes cut off subject lines so we need to add the rules here.

There are a lot of details, so it is worthwhile to use a tool that already knows the basics of Rule 3 so you can focus on the choices in Rule 2 that will give you the greatest possible lift. 

While you are testing your subject lines with an AI powered tool, keep in mind the following pitfalls: 

• Variation when regenerating response – any AI tool can get stuck on similar themes/words when refreshing the initial response.

• Boring or overused marketing language – Queries that are too general tend to generate equally general responses. The more specific or targeted you are with prompts, the fresher and cleverer the tool will be.

We asked our tool to write a few subject lines for the topic of this blog post. Here’s the list, and the projected winning line: 


“AI: the ultimate wingman?”


“AI’s secrets to email success

“Want to know what AI thinks?”

“The future of email is here”

“These AI tips will blow your mind”

We rather like AI as our wingman. 

Using AI to generate subject lines can help you save time, and using a tool like Subject Line Pro with AI will help you pick smarter winners. That can help you go back to testing the subject lines that will give you the lift you want, instead of letting subject lines test your patience.