[ In short ]
- Phones show roughly the first 40 characters of a subject, desktop inboxes about 60; put the words that matter first.
- For one-to-one business email, 3 to 9 plain words usually works: what it is about, and what you need.
- Capitals, "!!!", promotional words and fake "Re:" make a message look like marketing or a trick.
- These are rules of thumb. Whether mail reaches the inbox depends far more on your domain’s reputation and authentication.
01Check your subject line
Type or paste a subject. The checker counts characters and words, shows roughly how a phone and a desktop inbox would cut it, flags what reads as spammy or pushy, and gives a score with the reasons. Edit and watch the score move.
Phone · about 40 characters
Desktop · about 60 characters
Checks
- Good: 36 characters: shows whole on most phones.
- Good: No spammy words, capitals or punctuation tricks.
Rules of thumb, not a spam filter. Where a subject is cut depends on the app, screen and sender name; whether mail reaches the inbox depends mostly on your domain's reputation and authentication.
Runs in your browser. Nothing you type is sent anywhere.
02What the checker looks at
| Check | Why it matters |
|---|---|
| Length in characters | Between 15 and 45 shows whole on most screens and still says something. |
| Words | Fewer, plainer words are read faster in a crowded inbox. |
| Phone and desktop preview | Shows what the reader actually sees before opening. |
| Promotional phrases | "Act now", "free gift", "guaranteed": they read as an advert in a business thread. |
| Capitals | Whole words in capitals read as shouting. Short acronyms like CEO or Q4 are fine. |
| Punctuation and emoji | Repeated ! or ?, and more than one emoji, look like marketing. |
| Fake Re: or Fwd: | Pretending a message is a reply misleads the reader, and filters notice. |
| Merge tags | A missing fallback sends "Hi {{first_name}}" to a real person. |
| Question form | A clear question asks for an answer, which suits a follow-up. |
The score starts at 100 and loses points for each problem, more for the ones readers notice most. It is a quick sense check, not a prediction of opens or replies.
03What a subject line checker cannot tell you
Spam filters at Gmail, Outlook and Yahoo decide mostly on who is sending: the domain’s reputation, whether SPF, DKIM and DMARC pass, how recipients treated past mail, and complaint rates. A single word in the subject rarely sends a legitimate email to spam on its own. Word lists like the one here are useful because they sound like marketing to people, not because they trip a filter.
If your mail is landing in spam, start with authentication: build your records with the SPF record generator and the DMARC record generator, then work through the deliverability checklist.
Truncation varies too. The same subject is cut at different points in the Gmail app, Apple Mail and Outlook, and by the length of the sender’s name. Write so that the first few words carry the message.
04Subject lines for follow-ups that get replies
For a follow-up, the best subject is usually the original one: reply in the same thread so the history is right there. When you need a new subject, make it specific to the person and the next step.
| Instead of | Try |
|---|---|
| Just checking in!! | Question on the Q4 quote |
| URGENT: please respond | Can we confirm by Friday? |
| Following up | Data room for Wednesday’s partner meeting |
| Re: (on a new email) | Next steps for the design role |
| Don’t miss this exclusive deal | 15 minutes on Tuesday to walk through onboarding? |
More examples by situation are in email subject lines that get replies.
05Common subject line mistakes
- Putting the useful words last, where a phone cuts them off.
- A vague subject ("Hi", "Quick question") on a first email; fine in a thread, weak on its own.
- Changing the subject of an ongoing thread, which breaks it apart in many mail apps.
- Copying marketing habits (emoji, capitals, urgency) into one-to-one email.
- Merge tags without a fallback in templates.
Skein’s agent drafts follow-ups inside the existing thread, so the subject and the history stay intact. Try the demo to see a drafted follow-up.
Questions
- How long should an email subject line be?
- Short enough that the important part survives on a phone, which shows roughly the first 40 characters. For business email, 3 to 9 words is a good range. Longer can work if the first words carry the message.
- Do spam trigger words really send email to spam?
- Rarely on their own. Modern filters weigh sender reputation, authentication and how recipients react far more. The words still matter because people skim them as marketing.
- Should a follow-up email have a new subject line?
- Usually not. Reply in the original thread so the history is visible. Use a new subject only when the topic has changed or the original thread is very old.
- Is the subject line I type stored anywhere?
- No. The checker runs in your browser and does not send or save anything.
See Skein follow up for you
Business email on your own domain with an agent that follows up on every conversation. Hosted in the EU. Try the demo with sample mail, nothing to install.
[ Sources ]
Facts about other products were checked on 5 Oct 2026; prices and plans change.