The one line everybody reads
What it has to do, the length that actually matters, and what backfires.
6 min read
The subject line is the one sentence everybody reads. It does not decide whether your letter is good — it decides whether it is opened, and that is a different craft. Here is what works, what does not, and what is measured rather than guessed.
Say it, rather than sell it
A subject that honestly announces the content beats a clever one. « Your September invoice » has no elegance whatsoever, and it gets opened: the reader knows why it is there.
A mysterious subject works once. The second time, the reader has learned that the inside did not match the outside — and that is a reader you do not get back.
Length: what is shown, not what is written
The standard allows nearly a thousand characters. What is displayed depends on the mail client and above all on the screen: in practice an inbox shows forty to sixty.
The conclusion is not « keep it short », it is put the essential up front. A long subject is not a flaw; a subject whose first six words say nothing is.
The preheader, the second line everyone forgets
Under the subject, most inboxes display the beginning of the message. Left to chance, it shows « If you cannot read this message properly… » — the worst possible opening line.
Written on purpose, it is a free second line: the subject announces, the preheader specifies.
What backfires
What backfires
Capitals and exclamation marks. They trigger no filter — the myth of forbidden words has no support in providers' documents — but they tire people out, and fatigue is measured in unsubscribes.
The fake « Re: » or « Fwd: ». Pretending there was a conversation is the surest way to get reported rather than opened.
The broken promise. « Your gift is waiting » on a news update: the open is won, the reader is lost.
Test, rather than guess
The only way to know: try both
No rule replaces a test on your own list. Two subjects, part of the list for each, and the one with more opens goes to the rest.
One caveat that matters: below about thirty recipients per version, the gap means nothing. Better to declare no winner than to invent one.
A good subject says what is inside, puts the essential up front, and keeps its promise once opened. The rest is measured — on your list, not against an industry average.
Words from this guide
Two subjects, and the list decides
Plumail's A/B test sends two versions to part of your list, then sends the winner to the rest. Below thirty recipients per version it declares no winner — it says so instead of guessing.