See your subject line the way the inbox will
Most subject lines are written in a wide desktop field and read in a narrow mobile list. This shows where each major client cuts yours off, and flags the patterns worth a second look — with the reason attached, not a made-up score.
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Why there is no score out of 100
Plenty of subject-line tools return a confident number. That number is invented. No heuristic can predict an open rate, because opens depend overwhelmingly on who is sending, whether the recipient recognises them, and whether the last few emails were worth reading — none of which is visible in the subject text.
What can be measured is measured here: exact length, where real clients truncate, emoji and punctuation counts, capitalisation, and specific terms that correlate with mail people report. Each finding comes with the reason, so you can disagree with it when your context warrants.
Front-load the first 35 characters
iPhone Mail in portrait shows roughly 35 characters. The Gmail app shows around 45. Desktop Gmail reaches about 70. Since more than half of email is opened on a phone, anything past character 35 should be treated as reinforcement rather than message. The common failure is a subject whose meaning lives at the end — “Your order from Northfold Supply has shipped” truncates to “Your order from Northfold Su…”, which loses the only new information in it.
Preview text is the half most people waste
The line underneath the subject is preview text, and it occupies roughly as much display space as the subject itself. Leave it unset and the client fills it with whatever text appears first in your HTML — which is very often “View this email in your browser”. That is a significant portion of your inbox real estate spent on a utility link.
Use it to continue the thought rather than repeat it. If the subject is the headline, the preview is the standfirst: it should make the reader want the rest, not restate what they just read.
What “spam words” actually means in 2026
The idea that filters block on individual words has been out of date for well over a decade. Modern filtering weighs sender reputation, authentication, engagement history and complaint rate far above vocabulary. A trusted sender can put the word “free” in a subject line and land in the inbox; an unknown sender with poor authentication will land in spam writing about nothing in particular.
The reason the flagged terms still matter is indirect: they correlate with the kind of mail recipients mark as spam, and complaints are what genuinely damage you. Gmail expects bulk senders to stay under a 0.3% complaint rate. Language that pushes people toward the report button is a deliverability problem, just not the mechanical one the myth describes. The deliverability guide covers what actually moves inbox placement.
Test on your own list, not against a checklist
Every rule here has been beaten by someone’s A/B test. Subject lines are one of the cheapest things to test properly: split the list, send both, and let your own audience settle it. Use this tool to catch the mechanical problems — truncation, an empty preview line, four exclamation marks — and use testing for everything else.
Questions
How long should an email subject line be?
Long enough to be specific, short enough that the important part survives truncation. Practically: aim for the essential meaning inside the first 35 characters, because that is roughly what iPhone Mail shows in portrait. Total length between 30 and 60 characters is a reasonable working range, but a specific 70-character subject beats a vague 40-character one.
Do emoji in subject lines help?
Sometimes, and one is usually the limit. A single well-chosen emoji can help a message stand out in a crowded list. Three or more reads as promotional, and some corporate Outlook configurations render them as empty boxes — so the subject needs to work without them.
Will the word 'free' send my email to spam?
On its own, no. That has not been how filtering works for a long time. Sender reputation, authentication and engagement history dominate. What the word does is nudge some recipients toward reporting the message, and complaints above Gmail's 0.3% threshold genuinely do harm you. So it is worth using deliberately rather than reflexively.
What is preview text and where do I set it?
It is the line shown beneath the subject in most inboxes, sometimes called preheader text. Every email platform has a field for it, usually next to the subject line. If you leave it empty the client pulls the first text in your HTML instead, which is typically a 'view in browser' link — a waste of roughly as much space as the subject itself.
Does personalisation in the subject line improve open rates?
It can, and it is not automatic. A first name adds nothing if the rest of the subject is generic, and it actively hurts when the merge field is empty and the reader sees 'Hi ,'. Always set a fallback value. Personalising on behaviour — what someone actually did — tends to outperform personalising on a name.
Is my subject line sent to your servers?
No. The analysis runs entirely in your browser and nothing is transmitted or stored.
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