Here is the counterintuitive thing about using AI to write marketing emails. The shorter and more transactional the copy, the more likely AI will outperform you. The longer and more brand-voice the copy, the more likely a one-shot prompt will produce something flat, hedged, or quietly wrong. Stores that get great results from AI email aren’t using it as a writer. They’re using it as a draft engine for the parts where draft engines win, then running their human time where it actually moves revenue.
Shopify’s Commerce Trends 2025 reports roughly 8 in 10 merchants now use at least one AI feature, with content generation among the most common. The merchants getting real lift are running structured A/B tests, not vibes. This post is the test you can run this week, and the prompt chain to set it up.
The split: where AI wins, where it loses
Subject lines, preview text, abandoned-cart sequences, post-purchase confirmations, shipping updates, and short transactional product blocks all sit in AI’s wheelhouse. The information is concrete, the length is tight, the tone is mostly functional, and the model has read tens of millions of examples. You will write a passable subject line in 90 seconds. You will not write twenty variants in 90 seconds. AI will.
Brand-voice flagship campaigns, founder-letter sequences, story-driven welcome flows, and anything that depends on a specific shared joke with your audience are the opposite. The model has not read your last 200 emails. It has read everyone else’s. Use AI as the first draft and your hand will come off the wheel exactly when the copy needs to feel like nobody else could have written it. That is the failure mode that loses the open rate war over six months.
Run this A/B test before you change anything
The honest way to find out whether AI improves your email program is to test it on a campaign you have already sent. Pick a recent product-launch email that performed averagely (open rate within five points of your list average, click rate within two points). Then run a structured A/B test against three AI variants.
“You are writing for [STORE NAME], an eCommerce brand selling [PRODUCT CATEGORY] to [CUSTOMER DESCRIPTION]. Our voice is [3 ADJECTIVES] and our customers respond best to emails that [BEHAVIOUR, e.g. open with a specific scene, never use exclamation points in subject lines, end with a question]. Read the example email below and produce three rewrites under three different angles: variant A is benefit-led with a numerical specific in the subject line, variant B is story-led opening with a customer scenario, variant C is curiosity-led with the product reveal in the second paragraph. Each rewrite should keep the same offer, CTA, and approximate length as the original. Output as a table with columns: variant, subject line, preview text, body, CTA copy. EXAMPLE EMAIL: [PASTE].”
Send each variant to a 10% segment of your list, hold the original as the control on another 10%, and read the difference at 24 and 72 hours. Use Klaviyo or Omnisend to run the segment split. The test takes a week. The result is the only honest way to know which type of campaign in your program benefits from AI assistance and which does not.
The prompt chain for short copy at scale
Once you know AI is helping in a category, scale it with a chain rather than one big prompt. The chain trick is what separates merchants who write twenty subject lines in fifteen minutes from merchants who write two and stop.
Step one is the brief. Drop the campaign details into ChatGPT or Claude and ask for a structured brief, not copy:
“I’m sending an email for [CAMPAIGN NAME, e.g. our July warehouse-clearance event]. The audience is [SEGMENT, e.g. lapsed customers who last bought 90 to 180 days ago]. The offer is [OFFER, e.g. 30% off select summer styles, ends Sunday]. The single most important action I want them to take is [ACTION]. Output a one-paragraph creative brief I can paste into the next prompt, including target emotion, tone, must-include details, and three things to avoid.”
Step two is the bulk-generate. Paste the brief and ask for twenty subject-line variants under tight constraints:
“Using the brief above, write 20 subject lines for this campaign. Constraints: 35 to 55 characters, no exclamation points, no the word ‘just’, avoid clichés (don’t miss out, last chance, hurry). Mix five styles evenly: numerical specific, curiosity gap, direct benefit, customer-quote-style, and one-word punch. Output as a numbered list with character count after each.”
Step three is the cull. The pattern with bulk subject-line generation is that variants 1 to 5 are usable, 6 to 12 are middling, and 13 to 20 contain a couple of unexpectedly good ones. Read all twenty. Pick three. Run them as test sends.
The constraint list is the trick. Without it, the model averages toward the same five patterns it has been trained on. With it, the model produces variety inside a frame, which is exactly what you want.
The single biggest failure mode: brand-voice drift on autopilot
If you set up an AI flow that writes your weekly campaigns end to end without human editing, your voice will drift inside three months. The drift is invisible at the per-email level. It shows up in the aggregate as a slowly declining open rate, a flattening of the personality your subscribers signed up for, and an inbox that looks like every other store’s inbox.
The fix is mechanical, not philosophical. Once a quarter, take ten of your AI-assisted emails and ten of your founder-written emails from the last year, paste them into a single document, and ask:
“Compare these two sets of emails. Set A was written with significant AI assistance, set B was written entirely by the brand founder. Identify five specific markers of voice that appear in set B but are missing or weaker in set A. Then suggest three concrete additions I can include in every future AI prompt to bring set A back toward set B. Be specific (sentence-length patterns, vocabulary choices, openers, closers, recurring rhetorical moves).”
The output is the corrective for the prompt you have been using. It is the closest thing to a guardrail against silent voice drift, and it takes about 20 minutes per quarter to run.
The honest take on the original question
Can AI write better marketing emails than you. Sometimes. On a single rushed product-launch email, with twenty minutes of prompt iteration, the AI variant will often beat the version you would have shipped at midnight on a deadline. Across a whole program over twelve months, AI without your judgement and your editing hand on top of it will not. The merchants seeing real lift are running the split intentionally: AI as the bulk-draft engine for the categories where it wins, the founder or copy lead writing the campaigns that carry the brand.
The marketers in your space who are quietly compounding open rates over the next year are not the ones picking a side. They are the ones who know the line.