Email subject lines that get opened

Email subject lines that get opened come from rotating six proven categories (provocative, how-to, mistake, curiosity, story, why) and prompting AI with fill-in-the-blank templates from real winners, not with “write me subject lines”. Get this right and you have hundreds of on-voice subject lines plus a bottomless well of email ideas.

The first email I ever wrote for a client, in March 2020 for five dollars, came back with one note: “this is okay, but this subject line will never get opened.” Fair. The subject line is 50 to 70 characters that decide whether everything else you wrote exists. Here’s the system I built after that humbling.

Start with a swipe file (your taste is the tool)

Every podcast and book on this topic gives the same first advice, and it’s right: create an email address whose only job is collecting marketing emails from writers who make you click. Label them, study what caught you. AI can generate endlessly, but the swipe file is how you judge the output; without trained taste, you can’t tell the B+ lines from the duds, and you’ll ship duds.

From three years of my own swipe file, the best personality-driven senders rotate six categories:

Category	The move
Provocative	Poke the industry’s sacred cow
How-to	Named skill, concrete payoff
Mistake	”The error costing you X”
Curiosity	An open loop only the email closes
Story	A scene the reader must resolve
Why	The reasoning behind a stance

Everyone defaults to curiosity-plus-benefit; the rotation is what keeps your name feeling fresh send after send.

Prompt with templates, not requests

The difference between garbage output and grade-A output is one move: strip the context out of proven subject lines and hand the AI the skeletons. Instead of “write me 10 subject lines about email marketing”, the prompt includes patterns like “Warning: the dark side of [industry phenomenon]” and “[Specific skill] is half the battle. Here’s how to conquer the rest”, plus your niche. The model fills blanks instead of inventing clichés, and the brackets force specificity to your audience.

Two repair prompts for when quality sags:

• Punch-up: “Add a touch of National Enquirer to these.” Tabloid editors were the best subject-line writers in history; borrow their nerve, then tone to taste.
• Constraint: “No more than 70 characters.” AI drifts into magazine-headline length without it.

Run each of the six categories through the template prompt and you have hundreds of options per topic, in minutes.

The hidden bonus: subject lines are email ideas

Half the generated lines are really email premises: “Lessons learned: my evolution from [past state] to [current state]”, “My personal philosophy on [money/family/time]”, “Challenging the norm: how I changed my mind about [belief]”. Stuck at a blank screen, generate a category batch and you’ll find tomorrow’s email hiding in line seven. This feeds straight into the story→lesson→offer structure from the $100k newsletter playbook.

One warning: a subject line’s job is the open, and the open only pays if the email lands in the inbox at all (deliverability first) and the body delivers (the AI email system handles that). Don’t learn to catch fish one at a time; the whole rod-and-reel setup arrives weekly in the newsletter.

Frequently asked questions

How long should an email subject line be?

Under 70 characters, and shorter usually wins. AI tools tend to write magazine headlines, not subject lines; adding "no more than 70 characters" to the prompt fixes it instantly.

What is a swipe file and do I need one?

A dedicated email address subscribed to nothing but marketing emails from people whose writing you admire. It trains your taste, and taste is how you tell a B-grade AI subject line from an A-grade one. Every good email marketer I know keeps one.

Why do my AI-generated subject lines all sound the same?

Because "write me subject lines" gives the model nothing to vary against. Feed it templates with the context stripped out (fill-in-the-blank patterns from proven subject lines) and rotate through distinct categories, and the sameness disappears.

Do curiosity subject lines still work?

Yes, but curiosity-and-benefit is what everyone sends, so it wears out fastest. Rotating six categories (provocative, how-to, mistake, curiosity, story, why) keeps your name fresh in the inbox instead of triggering the "another one of these" reflex.