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Email subject line tester
Scored against the flow it belongs to. An abandoned-cart line and a winback line are doing opposite jobs, and judging both against “is it under 50 characters” is how a generic tester gives confident, useless advice.
What a subject line tester checks
The mechanics: length, whether the meaning survives mobile truncation at roughly 40 characters, capitals, punctuation, emoji and accumulated promotional language. What almost none of them check is fit — whether the line is right for the job the email is doing, which differs completely between a broadcast, a cart recovery and a shipping notice.
Test a line.
Prefilled with a line a generic tester would pass — and that is wrong for the flow it is in.
Return someone to a decision they already almost made.
The most valuable free space in the inbox, and usually empty. Roughly the first 90 characters show.
In this flow, a good line
- The specific item named
- A helpful, low-pressure tone
- Brevity — they know what this is
And avoids
- Urgency you cannot justify
- Discounting on the first touch
- Sounding automated
As it appears on a phone
Last chance! Your cart expires soon — 15…
55 characters · 11 words · truncated on mobile
0 to fix · 5 to check
- 55 characters OK In the range that displays fully on most desktop clients.
- Truncated on mobile Check Phones show roughly the first 40 characters. Yours reads “Last chance! Your cart expires soon — 15…” — check that carries the meaning on its own.
- Exclamation mark Check Fine in a broadcast. In a triggered email it makes an automated message sound more automated.
- 1 emoji OK Used sparingly. Check it renders on Outlook desktop, which handles them worst.
- Urgency in an abandoned-cart line Check Only justified if the stock or the price genuinely changes. Manufactured scarcity here trains people to ignore the flow entirely.
- Discount on the first touch Check Teaches people to abandon the cart deliberately. If a discount is used at all, it belongs on the third email, not the first.
- No preview text Check Left empty, most clients pull the first words of the email body — often “View this email in your browser”.
Scored in this tab. Nothing typed here is sent or stored.
What each flow is actually trying to do
The reason one rubric cannot serve all seven. These are our criteria, published so you can disagree with a specific line rather than a score.
| Flow | The job | Wants | Avoids |
|---|---|---|---|
| Broadcast campaign | Earn an open from someone who did not ask for this today. | A specific reason to open · Something true and new · Front-loaded meaning | Manufactured urgency · Discount-only framing · Vagueness dressed as intrigue |
| Abandoned cart | Return someone to a decision they already almost made. | The specific item named · A helpful, low-pressure tone · Brevity — they know what this is | Urgency you cannot justify · Discounting on the first touch · Sounding automated |
| Browse abandonment | Be useful about something they looked at, without being creepy. | Category-level relevance · A reason to return that is not pressure | Naming exactly what they viewed in a way that feels surveilled · Any urgency at all |
| Welcome | Set an expectation and make the first purchase easy. | Warmth · A clear next step · The brand's actual voice | Leading with a discount · Corporate throat-clearing |
| Replenishment | Arrive when they are running low and make reordering trivial. | Service tone, not sales tone · Timing that implies you know the product | Discounting a purchase they were going to make anyway · Urgency |
| Winback | Acknowledge an absence and give a reason to return. | A reason that is about them · Honesty about the gap · Something genuinely new | Guilt · “We miss you” as the whole idea · Escalating discounts as the only lever |
| Shipping and transactional | Convey status instantly. Highest open rate you will ever get. | The status, first · The order or item identifiable | Marketing language · Emoji · Anything that makes it look promotional |
Ours, from building these flows — not a published standard, and worth arguing with. The 40-character mobile figure is a working rule of thumb across clients and devices rather than a specification. Reviewed 2026-09-09.
On spam words, honestly
The most over-sold idea in email marketing, and the reason a lot of subject-line advice is superstition.
- One trigger word will not send you to spam Modern filtering weights sender reputation and recent engagement far above vocabulary. A single “free” in a line from a domain people open is a non-event.
- Accumulation is the actual signal Urgency plus capitals plus exclamation marks plus three discount words is a pattern, and patterns are what filters match. This tool flags the stack rather than the word.
- Your list hygiene matters more than your wording Sending to people who stopped opening a year ago will hurt placement more than any subject line ever could — which is a suppression problem, not a copywriting one.
- Nobody publishes the list No mailbox provider publishes its trigger vocabulary. Any tool presenting one as authoritative — including this one — is presenting a working set of recurring offenders. Ours is in the source, and it is ours.
What this cannot tell you
Whether it will get opened. Open rate is driven by sender reputation, send time, the relationship and — since privacy protection began inflating opens — partly by machinery rather than people. A subject line is one input among several, and not the largest.
Whether it is on brand. A rulebook cannot judge voice. A line that breaks three rules here and sounds exactly like you may well outperform a clean one that sounds like nobody.
How it renders everywhere. Emoji, in particular, differ by client and platform and occasionally fail entirely on Outlook desktop. Send yourself a test.
Which of two lines is better. That is a test, not a score — and for email it needs a real sample. The A/B test calculator will tell you whether the difference you saw was real.
Definitions
- Preview text
- The line clients show after the subject. Set explicitly, or the client picks the first words of your email body.
- Truncation
- Where the client cuts the subject. Around 40 characters on phones, more on desktop.
- Trigger word
- Vocabulary associated with promotional mail. Weakly weighted alone, meaningful in accumulation.
- Flow
- An automated sequence triggered by behaviour, as distinct from a broadcast campaign sent to a segment.
- Sender reputation
- The mailbox provider's running judgement of your domain and IP. Matters far more than any subject line.
Questions about subject lines
What makes a good email subject line?
It depends entirely on what the email is for, which is why a single score out of 100 is close to meaningless. A broadcast has to earn an open from someone who was not expecting it. An abandoned-cart email is talking to someone who already nearly bought — brevity and specificity beat persuasion. A shipping notice needs the status in the first three words and nothing else. Same rules applied to all three would be wrong twice.
How long should a subject line be?
Short enough that the meaning survives the first 40 characters, because that is roughly what a phone shows before truncating. Longer lines are not penalised by anything except the reader — but everything past that point should be a bonus rather than where the point lives. That 40 is a working figure across clients and devices, not a published standard.
Do spam words really matter?
Individually, almost never. Modern filters weight sender reputation and engagement far more heavily than vocabulary, so a single instance of “free” will not send you to spam. Stacked, they matter: a line carrying urgency, capitals, exclamation marks and three discount words is the pattern filters were built to catch. This tool flags accumulation rather than individual words.
Why does an abandoned-cart discount get flagged?
Because it teaches people to abandon carts. If the first recovery email reliably carries 15% off, a segment of your customers learns to add to cart and wait — and you have converted a full-price purchase into a discounted one you would have won anyway. If a discount is used at all it belongs late in the sequence, after the helpful touches have failed.
Does this use AI to rewrite my line?
No. Every tool on this site is deterministic — arithmetic or a parser — and this one is a rulebook applied to text. Nothing you type is sent anywhere, and the same line always gets the same result. If we ever add a rewrite feature it will query a model, and the page will say so in the first paragraph rather than the footnotes.
Why check preview text?
Because it is the most valuable free space in the inbox and most brands waste it. Left empty, clients pull the first words of the email body — which is frequently “View this email in your browser”. Repeating the subject wastes it differently. It should extend the subject, not echo it.
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