What decides whether a subject line gets opened
A subject line has one job: get the email opened. The reader sees it for a second or two, sitting among forty or fifty others in the same inbox, and decides in that time whether to open it, ignore it, or delete it. Most weak subject lines fail for one of four reasons: the line says nothing specific, it runs long enough that the point never reaches the phone screen, it manufactures curiosity instead of earning it, or the preview text beside it (the line most inboxes pull from the start of the email itself) wastes the second chance to make the case. The five rewrites below use the same brief throughout, so the changes can be compared directly: a promotional email to existing customers of a UK project-management tool, announcing that tasks can now be assigned straight from the inbox.
Five subject lines that get deleted, five that get opened
The subject line with nothing specific in it
Original: "Some big updates for you"
This could have come from any company, about anything. "Big updates" describes how the sender feels about the news, so the reader has nothing to weigh before deciding whether to open.
Rewrite: "Assign tasks straight from your inbox"
This names the actual feature and states it as something the reader can now do. The reader can judge, from the subject line alone, whether it is worth a click.
If a reader could delete a subject line without ever finding out what they missed, it was not specific enough.
The subject line that gets cut off before the point arrives
Original: "We've been working hard behind the scenes and we think you're going to really love what we've built for you" (107 characters)
The useful part of the sentence, what is actually being announced, doesn't appear until well past where most phone screens stop showing a subject line. Everyone reading this on a phone sees only the throat-clearing.
Rewrite: "Assign tasks from your inbox now" (32 characters)
The whole point sits inside the first four words, so it survives being cut off wherever the reader's inbox decides to cut it.
Write the point first, then check the length. Writing to a length and hoping the point survives the edit works the other way round, and it shows.
The subject line that manufactures curiosity
Original: "Something big is coming..."
The ellipsis promises information and withholds it. Open it, and it reads much like any other promotional email, with nothing to reward the click. The next time the same sender uses the same device, the reader recognises it and deletes without opening.
Rewrite: "Assign tasks from your inbox from Tuesday"
This still leaves a small gap (what changes on Tuesday, exactly), but the gap is a real detail withheld.
A curiosity gap works when the reader is missing one piece of a real story. It fails when the whole story is missing and the gap is doing all the work.
The subject line that fights its own preview text
Original subject: "Your March feature update", left with the default preview text: "This email may not display correctly. View it in your browser."
Preview text sits directly beside the subject line in most inboxes and is visible before the email opens. Leaving it as default boilerplate spends that visible space on nothing.
Rewrite: same subject, "Your March feature update", with the preview text set to "Assign tasks straight from your inbox, no separate app required."
The preview text now finishes the sentence the subject line started, so a reader scanning past sees a complete pitch.
Preview text is part of the same sentence as the subject line. Most email platforms let you set it separately from the first line of the body copy, so leaving it on default is a choice, even when it doesn't feel like one.
The subject line that borrows urgency it hasn't earned
Original: "Last chance! Offer ends tonight!!"
Exclamation marks and "last chance" turn up in enough promotional email that a reader recognises the pattern. If the account doesn't actually revert at midnight, the line is asking to be believed on a fact that isn't true, which costs more than the click it was chasing.
Rewrite: "Annual plan reverts to monthly on 30 April"
The date does the work "last chance" was trying to borrow, and it holds up if the reader checks.
A deadline that's real needs no exclamation mark. State the date.
Check your own subject line before you send
Length and specificity can both be checked without waiting for an email to go out and see what happens to it. Run your subject line through the character counter to see how much of it survives a phone screen, and read how to write email copy that gets opened and read for the fuller argument on preview text and the body copy that has to make good on whatever the subject line promised.