Score your subject line and get stronger versions.
A subject line should be scored on four things: length (roughly 35 to 45 characters before mobile truncation), specificity, spam risk, and whether the email body actually delivers what the subject promises. Open rate alone is no longer a reliable measure because privacy features pre-load images and inflate it.
Attach a PDF or doc (optional) for context. AI-generated, so give it a quick read. Free, no signup.
A free email subject line tester for sales and marketing teams. Paste a subject line (and the audience or context if you have it), and it gives a quick score, a short breakdown of what works and what to fix, and five to seven stronger versions built to get opened on mobile. Free with no signup.
Real results from this tool, so you can see what you get before you type anything.
Length. Mobile clients typically display around 35 to 45 characters. Write for that ceiling, because a subject line whose meaning depends on the last few words will lose it for a large share of recipients.
Specificity. The single strongest signal that an email is worth opening is evidence it was written for one person. A detail only true of the recipient beats any amount of clever phrasing.
Register. Subject lines that look like internal email outperform ones that look like campaigns, because the reader is scanning for things that belong among messages from colleagues. Lowercase and unpunctuated helps here.
Promise alignment. The subject line sets an expectation the first two sentences must honor. A curiosity gap that resolves into a standard pitch produces an open and a lost prospect, which is worse than no open.
Open tracking works by loading a tiny invisible image. Privacy features now pre-load those images on behalf of the recipient, which registers an open whether or not a human ever looked at the message. A meaningful share of reported opens are machines.
The practical effect is that open rate is inflated unevenly, depending on what email clients your audience uses. Comparing this quarter's open rate to last year's, or your rate to an industry benchmark, is comparing two differently distorted numbers.
Judge subject lines by reply rate for cold outreach, or by click rate for marketing email. Both require a human. If you must use open rate, use it only to compare two variants sent to the same audience in the same window, where the distortion is at least consistent.
Modern filters evaluate many signals together, including sender reputation, authentication, engagement history, and content. No single word gets you filtered on its own, which is why "avoid the word free" advice is only half right.
That said, certain patterns raise your score for no benefit: all caps, multiple exclamation marks, currency symbols, excessive punctuation, and heavy promotional phrasing. There is rarely a reason to include them.
The larger factors are usually technical and behavioral rather than lexical: whether your domain is authenticated, whether recipients engage or complain, and whether you are sending volume from a new domain. A perfect subject line cannot rescue poor sending infrastructure.
Change one thing at a time. If you test a new subject line and a new opening paragraph together, you learn nothing about either.
Give the test enough volume to mean something. Comparing two subject lines across 40 sends produces noise you will mistake for signal. If your volume is low, treat subject line choice as a craft judgment rather than a statistical one, and spend your testing budget on the offer instead.
Then check the body honors the subject. This tool scores the line, but the line is a promise. Our cold email writer drafts the body, and the subject line generator produces variants worth testing.