What the Email Subject Line Tester does
This email subject line tester checks up to 20 subject lines side by side: length, where each is cut off in a phone inbox and a desktop webmail row, capitals and punctuation habits, emoji, broken personalisation tags, fake RE: and FW: prefixes, and pushy or spam-like wording. Each finding is listed on its own, so you can see exactly why one variant looks better than another.
It gives no single score and makes no prediction about open rates, because no tool can know your audience. The rule against deceptive subject lines is law in the US and part of Yahoo's sender guidelines; everything else here is readability advice and an A2Z heuristic phrase list, labelled as such.
How to use it
- Type your subject line ideas, one per line.
- Add the sender name and preheader you plan to use, so the inbox previews show the full picture a reader sees.
- Compare the variants in the table: characters, problems, warnings, and whether the subject fits on one line on a phone. Press Show to see any variant in detail.
- Fix the problems first - a fake reply prefix or a broken merge tag - then consider the warnings.
- Test the finalists with a real A/B test in your email platform; this page helps you choose which ones are worth testing.
Reading the results
Problems (red) should always be fixed: a subject that pretends to be a reply or forward is deceptive, and a merge tag such as {{first_name} with a missing brace is sent to people literally.
Warnings (amber) are habits that make an email look like spam to people and, sometimes, to filters: mostly-capital subjects, !!! and $$$, three or more emoji, and phrases from the trigger list.
The previews are approximations measured in your browser's font. Inbox apps truncate by pixel width, so a subject with wide letters is cut sooner than one with narrow letters of the same length.
Worked example: three subjects for an autumn sale
"Autumn sale: 20% off jackets and boots" is 38 characters with no issues, and the offer is in the first few words.
"Our autumn sale starts today - and it includes the wool jacket you looked at last week" is 86 characters. Nothing is wrong with it, but on a phone it is cut off after the first few words and the personal hook at the end is never seen.
"Priya, your 20% off ends Sunday" is 31 characters with a personal name and a deadline. The spam-habits example shows the opposite: "RE: ACT NOW!!! 100% FREE $$$" with three emoji has one problem (the fake RE:) and five warnings.
Do spam trigger words still matter?
Much less than they used to. Large mailbox providers filter mainly on sender reputation - authentication, complaint rates and how recipients engage with your mail. A single word such as "free" will not send a message to spam on its own.
The phrases on this page's list still matter to readers, who have learned to associate "ACT NOW" and "You've been selected" with scams, and they add to the case against a message that already has other problems. That is why they are shown as warnings with a reason, not as a verdict.
Limitations: what the result does not prove
- It cannot predict open rates. Only a test with your own list can.
- Preview widths are approximate, and inboxes differ by app, device, font size and whether the preheader is shown.
- The trigger-phrase list is a short A2Z heuristic, in English only; it is not any mailbox provider's filter.
- Personalisation tags are checked for syntax only; whether your platform has a value or fallback for each contact is not something this page can see.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- US Federal Trade Commission - CAN-SPAM Act: a compliance guide for business
- Yahoo Sender Hub - Sender best practices
- Google Workspace Admin Help - Email sender guidelines
Frequently asked questions
How long should an email subject line be?
Long enough to say what the email is, short enough that the key words are visible on a phone - often around 30 to 50 characters, though pixel width matters more than character count. The previews show where each variant is cut so you can put the important words first.
Is it illegal to put RE: in a marketing email subject?
In the US, the CAN-SPAM Act prohibits subject lines that mislead recipients about the content of the message, and a fake RE: or FW: on a message that is not a reply or forward does exactly that. Yahoo's sender guidelines also ask senders not to use deceptive subjects. The tester flags these prefixes as problems.
Do emoji in subject lines help or hurt?
It depends on your audience and brand. One relevant emoji can add colour; several read as noise, and some older clients show them as empty boxes. Screen readers read them out by name. The tester notes any emoji and warns from three.
What happens if a personalisation tag breaks?
If a merge tag is malformed, such as {{first_name} with a missing brace, most platforms send it as plain text, so readers see the code. If the tag is valid but the contact has no value, the subject may read "Hi ,". The tester catches broken syntax; set a fallback value in your platform for the second case.
Which words make emails go to spam?
No word does it on its own. Filters at Gmail, Outlook and Yahoo rely mainly on authentication, sender reputation and how people react to your mail. Aggressive wording still hurts because readers ignore or report it, and complaints do affect delivery. The warnings here point out wording that readers commonly distrust.
Should the preheader repeat the subject line?
No. The preheader is the second line of the inbox preview, so repeating the subject wastes it. Use it to add a detail, a benefit or a deadline the subject did not have room for. The tester warns when the two say the same thing.
Last reviewed by the A2Z.Tools team against the sources listed above.
