Email subject lines: how to write ones that survive the preview pane

Subject line advice is mostly folklore, because the thing everyone optimised for — open rate — stopped being measurable when Apple began pre-fetching images. What remains true is structural: how much text is displayed, what sits next to it, and whether the promise it makes is kept by the email.

Those you can control without guessing.

Write for the space you actually get

Desktop clients show more; phones show less, and most of your list is on a phone. Assume the first 35–45 characters are what gets read, and that everything after may as well not exist.

• Front-load the specific thing. The product, the number, the deadline — not the wind-up.
• Brand name in the subject is usually waste, because the sender name is already displayed beside it.
• Avoid all-caps and stacked punctuation. They do not raise opens and they do raise the chance of being treated as promotional.
• Emoji are neutral at best. They render inconsistently, they consume display width, and in some clients they are stripped, leaving a gap.

The preheader is the second half of the subject

Treat subject and preheader as one sentence split in two. The subject makes the specific promise; the preheader adds the detail that makes it worth opening.

1. Write it explicitly, every time, in a hidden preheader block at the top of the email.
2. Do not repeat the subject. Two identical lines waste the only extra space you have.
3. Keep it under about 90 characters, and pad after it with a hidden zero-width space so the client does not pull body text in behind it.

The four that keep working

1. Specific and concrete. "Your size is back in the linen shirt" beats "New arrivals you'll love". Specificity is the only reliable lever.
2. A real deadline. Only if it is real. Manufactured urgency works once and costs you the sender reputation of the next twenty campaigns.
3. A genuine question the reader has. "Does it run small?" works because the buyer was already wondering.
4. Plainly useful. "Your order arrives Thursday" is not a marketing subject line, and it is the one that gets opened every time. Borrow its register more often.

Testing it honestly

Most subject line A/B tests prove nothing, because the lists are too small for the difference being measured. A 2 % difference on 2,000 recipients is noise.

• Test on revenue per recipient, not on opens. Opens are not reliably measurable any more and were never the goal.
• Test big differences, not synonyms. Question versus statement, specific versus broad. Small variations need sample sizes you do not have.
• Keep a record of what you learned, in a sentence, per test. Most shops rerun the same test every year because nobody wrote the answer down.
• Check rendering before you check cleverness. Send the campaign to yourself on a phone and a desktop client and look at the list view, not the open message.

The subject is a promise the email has to keep

The fastest way to train a list to ignore you is a subject that overstates what is inside. It earns an open once, then a slightly lower open rate for ever afterwards, and eventually a complaint from somebody who feels misled.

Before sending, read the subject and then the first screen of the email as a stranger would. If the first screen does not immediately deliver what the subject promised, change one of the two. Usually the subject was right and the email buried the thing.