Email Subject Line Tester - Length, Spam Words, Preview

What this tester checks

A subject line has one job: get the right people to open. The tester looks at the things you can control and that measurably affect that: how much of the subject survives truncation on a phone, whether it reads like hype, whether the preview text is doing any work, and whether the sender name is recognizable. Each check is shown as a pass or fail, and the score is a simple sum of penalties so you can see exactly why it moved.

What it cannot tell you is whether your audience will care. No tool can. The score catches mistakes; an A/B test on your own list tells you which subject wins.

Length: design for the phone

Litmus and other email analytics firms have reported for years that mobile accounts for somewhere around 40 to 60% of opens, depending on the audience, and consumer lists skew higher. On a phone in portrait mode, the iPhone Mail and Gmail apps typically show about 35 to 40 characters of the subject before an ellipsis. Desktop Gmail shows more, but the subject and preview text share one line of roughly 60 to 70 characters.

So front-load. "Your October invoice is ready" (29 characters) survives anywhere. "We wanted to let you know that your October invoice is ready" (60) gets cut to "We wanted to let you know that your Oc…" on a phone, and the useful part disappears. The preview box in the tester cuts at 40 characters to show roughly what a phone reader sees.

Worked example: rewriting a weak subject

Start with "FREE SHIPPING!!! Limited time offer - ACT NOW". The tester flags three spam phrases (free, limited time, act now) for 30 points, all-caps words for 10, repeated exclamation marks for 10, and a length of 45 characters, just past what fits on a phone, for 5 more. Score: 45.

Rewrite: "Free shipping on everything through Sunday" (42 characters). It still says free, so it loses 10 points and 5 more for length. Tighten to "Shipping's on us through Sunday" (31 characters), add preview text of "No minimum. Ends 11:59 pm PT.", and every check passes. Same offer, calmer tone, specific deadline, fully visible on a phone.

Spam words: what actually matters

Lists of "spam trigger words" are mostly folklore. Gmail, Outlook, and Yahoo filter primarily on sender reputation, authentication, complaint rates, and how recipients engage with your past mail. A single "free" in a subject from a well-authenticated sender with a clean list will land in the inbox.

The phrases still matter for a different reason: they read as hype, and hype lowers opens and raises complaints, which then hurts reputation. Since February 2024, Google and Yahoo require bulk senders (over 5,000 messages a day) to authenticate with SPF, DKIM, and DMARC, support one-click unsubscribe, and keep spam complaint rates below 0.3%. Fix those first; wording is the second-order problem.

Preview text and sender name

The inbox shows three things: sender name, subject, and preview text. Leaving the preheader blank wastes a line. The inbox fills it with the first text in the email, which is often "View this email in your browser" or an image's alt text. Write 40 to 130 characters that extend the subject rather than repeat it.

The sender name is the first thing people read and the biggest factor in whether they trust the email. Use a name people recognize, such as your brand or a real person at the brand ("Sam from Mavis Tools"), and keep it consistent from send to send.

Common mistakes

Clickbait that the email does not deliver on. It works once, then trains people to ignore you.

Fake "Re:" or "Fwd:" prefixes. Under the CAN-SPAM Act, subject lines must not be misleading about the content of the message, and the FTC can fine up to $53,088 per violating email as of 2025.

Testing only one version. Most email platforms, including Mailchimp, Klaviyo, and HubSpot, support subject line A/B tests. Send two variants to 10 to 20% of your list, then send the winner to the rest.