Email Subject Line Checker

Short enough to survive the smallest screen that matters to your list. Gmail on a phone shows roughly 40 characters before it cuts off, Apple Mail on an iPhone about 45, and desktop clients between 60 and 70. If more than half of your opens happen on mobile, which is common for B2B lists read first thing in the morning, put the point of the subject line in the first 40 characters and let anything after that be a bonus for desktop readers. There is no evidence that shorter is always better; the evidence is that the front of the line has to carry the meaning.

Less than they used to, and more than zero. Modern filters at Gmail and Microsoft weigh sender reputation, authentication, engagement, and complaint rates far above individual words. A subject line with the word free will not by itself land a well-authenticated sender in spam. But the words on this list correlate with the mail that does get filtered, and stacking several of them, particularly with all caps, multiple exclamation marks, and a currency figure, still hurts. Treat the scan as a hygiene check, not a verdict.

No, and be wary of tools that claim to. Open rate depends on the sender name, the list, the time of day, what the last five emails were like, and how much of the audience uses Apple Mail. The score here is rule-based: it starts at 100 and subtracts 10 for every deterministic check the subject line fails. Two subject lines with the same score can perform very differently. Use it to catch mechanical problems before a send, then test the actual copy with a subject line split test on a slice of the list.