What does the subject line tester do?
The subject line tester from Questry AI scores a subject line from 0 to 100 on six rules: the number of words, the number of characters, spam words, capitals and exclamation marks, a fake Re: or Fw:, and whether the subject line is about the recipient. You also see how the subject line looks in a phone inbox. The generator fills fixed patterns for cold B2B email with what you type, such as the job opening or the news you are writing about, and tests each subject line at once. There is no AI behind it: everything runs in your browser, and the same input always gives the same result. It is the English version of our Dutch onderwerpregel-tester, with the same rules.
How the score works
Each rule is worth a fixed number of points, 100 in total. Spam words count the most (25 points), then the number of words and the formatting (20 each), a fake Re: (15), and the number of characters and whether the subject line is about the recipient (10 each). A rule marked Good gives all its points, a rule marked Check gives half, and a rule marked Fix gives none.
A score of 85 or more is a strong subject line. Between 60 and 85 it can be better. Below 60 it pays to start again.
Why short and about the recipient
The studies do not agree on the best length. Hunter saw the most replies at 5 to 6 words in 31 million cold emails, Boomerang at 3 to 4 words, and Gong the most opens at 1 to 4 words. Backlinko saw the best response rate at 36 to 50 characters in 12 million outreach emails. They do agree that short works better. The tester therefore counts 2 to 7 words and at most 50 characters as good.
A personalized subject line gave 30.5% more replies at Backlinko. A subject line that names the role, the company or the reason shows that the email was written for one person. A fake Re: or Fw: pretends there was earlier contact. Google's sender guidelines say to use it only for a real reply or forward.
What is different for English lists
The rules and the limits are the same as in the Dutch tester. The spam word list has English and Dutch words, such as free, guaranteed, act now and click here, so the tester flags both. The full list, with a checker and as a CSV file, is on our spam word list page, in Dutch.
For the recipient rule the tester counts you and your, a field such as {company}, or a name with a capital letter. In title case, where every longer word starts with a capital, a capital does not count as a name.
How the generator works
The generator knows five reasons to write: a job opening, company news, a similar customer, a short question and the last email in a sequence. Each reason has fixed patterns, such as "Your opening for {role}" or "Wrong person?". What you type takes the place of the field. Every subject line goes through the same tester, and the best one is at the top.
Sources
We opened and read every source on 12 November 2026.
- Hunter, The State of Email Outreach 2026: Over 31 million cold emails sent in 2025: 4.5% replies on average, 3% in sales. Without personalization 3.6% replies, with two custom fields in the email 5.6%. Subject lines of 5 to 6 words got the most replies. Without open tracking 7.4% replies, with open tracking 4.4%.
- Boomerang, a study of 40 million emails: Over more than 40 million emails, not only cold ones: subject lines of 3 to 4 words got the most responses, not counting Re: and Fwd:. Emails of 50 to 125 words did best. Emails with 1 to 3 questions were 50% more likely to get a response than emails without a question.
- Gong, cold email to executives (2026): Open rates fall as subject lines get longer; subject lines of 1 to 4 words did best. The best cold emails have 50 to 100 words.
- Backlinko and Pitchbox, a study of 12 million outreach emails: Over 12 million outreach emails for link building and content, not for sales: subject lines of 36 to 50 characters got the best response rate. A personalized subject line gave 30.5% more replies.
- Google, Email sender guidelines: Gmail looks mainly at authentication (SPF, DKIM and DMARC) and a spam rate below 0.3%, with below 0.1% recommended. Do not start a subject line with Re: or Fwd: unless the message is a real reply or forward.
- Apache SpamAssassin, tests performed: An open-source spam filter that also scores the text, for example a subject in capitals (SUBJ_ALL_CAPS) and the words "100% guaranteed" (GUARANTEED_100_PERCENT).
Questions about the subject line tester
How long should a subject line be?
Short. The studies name 3 to 4 words (Boomerang), 5 to 6 words (Hunter) and 36 to 50 characters (Backlinko). The tester counts 2 to 7 words and at most 50 characters as good. Put the most important word first.
Can I use Re: in a cold email?
Only when it really is a reply in the same thread, such as a follow-up. A first email with Re: or Fw: pretends there was earlier contact, and Google's sender guidelines ask senders not to do that. The tester marks it as Fix.
Is the generator AI?
No. The generator fills fixed patterns with what you type. You know exactly where each subject line comes from, and the same input always gives the same subject lines.
Is the tool free?
Yes, without an account and without a limit. What you type stays in your browser.
Is there a Dutch version?
Yes. The onderwerpregel-tester uses the same rules with Dutch text and Dutch patterns, for Dutch lists.
Last updated on 12 November 2026.