Seven data-backed subject line rules, and the four that survive

There is a genre of email advice built entirely on open-rate data: put a number in it, add an emoji, keep it under 24 characters, use these words and never those. The studies behind it are real and the sample sizes are enormous — one of them covers 115 million emails.

They were also, almost without exception, run on bulk marketing email: newsletters, promotions and campaigns sent to people who had already handed over an address. That is a different situation from the one you are in when you send a five-line note to a facilities manager who has never heard of you. Some of the findings transfer cleanly. Some of them invert.

This is the whole list, what each rule is actually evidence of, and which four are worth following when your send volume for the week is eleven.

Where the numbers come from

The canonical roundup is Campaign Monitor's seven data-backed ways to write email subject lines, which pulls together several separate studies. Stripped to the claims:

Read the third column and the shape of the problem appears. Six of the seven are measurements of what makes a promotional email get opened by somebody who already agreed to receive promotional email. That is a real question. It is not your question.

The two hundred words

The two word lists are the most useful part of the research and the part everyone summarises instead of publishing. Here they are in full, both hundreds, so you can actually check a line against them.

Scan them and something is off. The lift list is mostly ordinary vocabulary: about, add, advice, can, do, this, you, your, tell, thanks, what, ways. The drag list is a catalogue of sales language: best price, incredible deal, risk free, why pay more?, join millions, this isn’t spam.

What the lists actually measure

Now put them side by side, and the thing worth knowing falls out. Words that mean nearly the same thing sit on opposite lists.

The first row is not a typo. “Congratulations” appears on both hundreds. A word cannot both raise and lower open rates, which means the lists are not really measuring words. They are measuring the sentence the word was sitting in.

“Congratulations on the Riverside contract” is a fact about a specific recipient. “Congratulations! You’ve Been Selected” is a form letter wearing a party hat. Same word, opposite outcomes, and no lookup table can tell them apart.

Which makes the “use three or more power words” advice actively bad for you. Deliberately stacking exclusive, limited time and last chance into one line produces exactly the register the drag list is made of. The lift words score well because they show up in emails that were already specific, not because they cause specificity.

Length is the rule with no argument

Rules five and six are the two that need no interpretation, because they are not psychology. They are screen width.

• Roughly half of all email is opened on a phone. That share is higher, not lower, for owner-operators reading between jobs.
• An iPhone shows about 35–38 characters of a subject line in the list view. A Galaxy shows about 33.
• So anything past the low thirties is not a subject line. It is a sentence the reader will never see the end of.

The research recommends 17–24 characters, which is tighter than it sounds — about three or four words. “Question about your Riverside site” is 34 and already at the cliff edge. “Quick question about the parking lot at your Riverside location” is 63, which means it arrives as “Quick question about the parking lo…” and the only part that was specific to them has been cut off.

This is also where rules five and six quietly fight rule three. Twenty-four characters, three to five words, three or more power words, and a number, all in the same line, is not a set of guidelines. It is an over-constrained system, and the thing sacrificed to satisfy it is always the specific detail — which is the only part that was working.

Numbers, emoji and title case

The remaining three rules are where bulk-email advice does the most damage to a cold local email, and they fail in different ways.

Numbers: keep it, but only real ones

The 206% figure comes from research on headlines — the listicle effect, where “7 ways” beats “some ways” because it sets an expectation of scope. A subject line is not a headline, but the underlying reason does carry: a number is a commitment to specificity, and it is hard to fake one.

The catch is that the number has to be theirs, not yours. “3 quotes in 24 hours” is a claim about your process. “Your 2 Elm St units” is a fact about their building, and it is the second kind that makes an owner open a message from a stranger.

Emoji: this one inverts

The 56% lift is real, and it is a consumer-brand finding — retail, travel, subscription boxes, sent to lists that opted in. Nothing in that population resembles a property manager receiving an unsolicited note about their nightly cleaning contract.

Owners who send this kind of email tend to arrive at the same place on their own: the note has to look like something a busy person typed on a phone between two jobs, because that is what it is. An emoji is the fastest available way to say it was not.

Title case: drop it

Title case tests well for the same reason it fails here. “Perceived authority” is what a publisher wants and precisely what a stranger asking for ten minutes does not. Pack My Box With Five Dozen Liquor Jugs reads as a headline somebody composed. Pack my box with five dozen liquor jugs reads as a sentence somebody wrote.

Nobody capitalises every word in an email to a person they know. Doing it in a first email announces that this is not that.

What survives at eleven sends

Set against a two-person company sending a dozen well-chosen emails a week rather than a campaign to forty thousand, the seven rules sort cleanly.

Four kept, three dropped — and the four that survived are the four that were never really about persuasion. They are about fitting on a screen and not sounding like a campaign.

How to write one

The subject line is not a separate craft from the email under it. If you have done the work described in what actually goes in the first email — found the one specific thing you noticed about this business — then the subject line is just that thing, stated flatly, in under thirty characters.

Three questions, in order, and they are quicker than running any checker:

• Could this line have been sent to any other business on the street? If yes, it is a campaign subject and it will be read as one.
• Does the specific part survive the first thirty characters? If the detail is at the end, the reader gets the generic half.
• Would you type it this way to somebody you know? If not, the capitals, the exclamation mark or the offer language is the reason.

The seven rules are worth knowing, because they describe accurately what bulk email looks like. For the kind of sending this newsletter is about, that makes them most useful as a list of things to avoid resembling.