Subject lines are not truncated at one universal point. What gets cut depends entirely on where it is read:
| Context | Approximate visible characters |
|---|---|
| iPhone Mail, portrait | ~35–40 |
| Android Gmail app | ~40–50 |
| Gmail desktop, inbox list | ~60–70 |
| Outlook desktop, reading pane | ~70–90 |
| Apple Watch | ~20 |
Since a majority of email is now opened on mobile, the practical target is under 40 characters, with the essential words in the first 30. Not because long subject lines are forbidden, but because everything after the cut may as well not exist.
This makes front-loading the single highest-value edit. Following up on our conversation about the Q4 pricing proposal shows as Following up on our conversation ab… — which is content-free. Q4 pricing: decision needed by Thu is 32 characters and fully visible.
| Pattern | Problem |
|---|---|
| ALL CAPS | Reads as shouting; correlates with spam scoring |
| Multiple exclamation marks | Strong spam signal, reads as desperate |
| Free, guaranteed, act now, limited time, risk-free | Classic commercial trigger vocabulary |
| Fake Re: or Fwd: prefixes | Deceptive; in the US, misleading subject lines breach CAN-SPAM |
| $$ , 100%, dollar amounts in caps | Filter-sensitive punctuation clusters |
| No subject at all | Frequently filtered outright |
| Excessive emoji | Render inconsistently; one can help, three do not |
A caveat on spam words: modern filters are statistical and reputation-based, not keyword lists. Sender authentication, complaint rate, engagement history and list hygiene matter far more than whether you used the word free. The old advice to avoid a fixed vocabulary is dated — a single trigger word in an otherwise well-authenticated email from a trusted sender is not what puts you in a spam folder.
Specificity. Concrete beats clever. Invoice 4471 overdue by 12 days outperforms Quick question for business mail, because it tells the reader what happens if they ignore it.
A reason to act now. A real date beats manufactured urgency, and it survives repetition — manufactured urgency does not.
Curiosity gaps, carefully. They work once. If the email does not deliver what the subject implied, the next one is ignored. Long-run open rate is a trust metric.
Matching the body. The most damaging pattern is a subject line optimised in isolation from the content. Opens go up, replies go down, and unsubscribes go up.
This is the part most subject-line advice omits, and it invalidates a great deal of published benchmark data.
Open tracking works by embedding an invisible image and recording when it loads. In 2021 Apple introduced Mail Privacy Protection, which pre-loads those images through a proxy for all Apple Mail users regardless of whether anyone read the message. Since Apple Mail is a very large share of email clients, a substantial fraction of recorded opens are now machine opens.
The consequences are concrete: reported open rates jumped without any change in behaviour; comparisons to pre-2021 benchmarks are meaningless; and A/B tests decided on open rate can now be won by the variant with the higher proportion of Apple users rather than the better subject line.
What remains reliable is downstream: click-through, reply rate, and conversion. If you are optimising subject lines, measure them against those instead.
Any scoring tool applies generic heuristics to a specific audience. Benchmarks differ enormously by industry, list quality and relationship — a transactional email to existing customers behaves nothing like cold outreach. Personalising with a first name was once a reliable lift and has largely been neutralised by ubiquity, and it actively backfires when the merge field fails. The only genuinely trustworthy method is testing on your own list, with a sample large enough to distinguish a result from noise, measured on a metric that has not been corrupted.
Yes, and it is badly underused. Most clients show 35 to 90 characters of preheader after the subject. Left unset it defaults to whatever your email opens with, often an unsubscribe link or a view-in-browser line.
Sometimes, because they invite a mental response. Overused they read as a formula, and a question the reader cannot answer is worse than a statement.
Sentence case generally reads as more personal and less like marketing, which suits business correspondence. Title case suits newsletters and announcements.
No. Whatever you paste is analysed in your browser and nothing is transmitted or logged.
Every tool comes with a written guide, and every category is one click away.