The subject lines that get cold emails opened are short, lowercase, and about the person reading, not about you. Think of the subject line you'd put on a note to a coworker: "quick question about the Elm Street job" gets opened. "Revolutionary solution to boost your ROI" gets deleted.
Below are twenty plain subject lines grouped by what you're doing, the rules behind them, and how to tell whether one is better than another without fooling yourself with numbers.
What a subject line has to do
It has one job: make the person willing to read the first line. It doesn't need to explain your offer or create urgency. If it makes people curious about the email, or looks like it belongs in their inbox, it has done its work.
The inbox is a list of names and short phrases, mostly on a phone. Subject lines that look like they came from a person tend to blend in with real mail. Subject lines that look like they came from a marketing team tend to be treated as marketing. That is common sense, not a measured law. Nobody has a public dataset that proves what works for your buyers, which is why the last section here is about testing your own.
Keep it under five words
On a phone, a long subject gets cut off after a few words. Aim for two to five, and put the useful word first. Lowercase looks like a note from a person; Title Case Looks Like A Newsletter. Skip punctuation tricks such as exclamation marks, all caps, and fake reply prefixes like "Re:" or "Fwd:" on a first email. Those are deceptive, and the law in the US (CAN-SPAM) requires that subject lines not mislead the reader about what's inside.
A subject line also has to match the email. If the subject says "quick question" and the body is a five-paragraph pitch, you've broken the promise in the first line.
Subject lines that name their business
The most dependable pattern is to put something specific to the reader where they can see it. Their company name, their street, the thing they sell. A subject line with their business in it is clearly not a mass mailing.
Rosa runs a small HVAC maintenance company and is emailing office managers in her town. Here are lines she might use:
- rooftop units at Calder & Finch
- question for the office manager
- spring tune-ups, Maple Plaza
- your building on Dock Street
- heating contract renewal?
Notice none of them says "offer" or "savings" or "opportunity". They read like a question from a neighbor with a trade.
Subject lines for a person you've never met
When the only thing you know is their role, a plain question works. These suit a first email where you haven't got a detail to point at yet, though a detail is always better (see personalization at low volume).
- quick question
- who handles payroll there?
- are you the right person?
- question about your fleet
- one thing about your site
"Quick question" is overused enough that it can start to feel like a template. It still works when the first line of the email delivers on it with an actual question.
Subject lines for a follow-up
A follow-up should usually stay on the same thread, using the original subject, so it stacks under your first message and shows the history. In Gmail that means replying to your sent email instead of composing a new one. For the second and third touches you can add a short line that signals what's new:
- re: rooftop units at Calder & Finch (reply on the same thread, so Gmail adds this for you)
- one more idea for Maple Plaza
- closing the loop
- should I stop emailing?
- last note from me
"Re:" is fine on a real reply to your own earlier message in the same thread. It is not fine to invent it on a message that isn't one. The follow-up wording and timing are laid out in a cold email follow-up sequence that does not nag.
Subject lines to avoid
A short list of what to cut, with the reason for each.
- Anything with "free", "guaranteed", "act now", "limited time" or dollar signs. These read like mass mail, and some filters do weigh them.
- "Following up" as the entire subject on a first email. You haven't been in touch yet.
- Fake urgency or fake familiarity: "Your account", "Invoice attached", "Great meeting you". If it's not true, it's deceptive and it'll sour the reply.
- Emoji and brackets. They sit oddly in a note from one business owner to another.
- Your whole pitch in the subject. If the subject says everything, there's no reason to open.
None of those words is magic poison. A subject line containing "free" won't doom a message by itself, since spam filters judge many signals together (how mailbox providers judge a sender explains the bigger picture). But there's no benefit in using words that make a stranger suspicious.
Test two at a time
You can compare subject lines, but be modest about what you can learn. Take your list of 60 and split it in half at random. Send version A to 30 and version B to 30, on the same day, with the same body. Count replies.
At those numbers, a difference of one or two replies could be luck. Treat a test as a nudge toward a better guess and rerun the winner against a new challenger, instead of declaring a champion. How to test a cold email with only 50 prospects goes through how to read the result honestly.
Why you should not chase open rates
It's tempting to compare subject lines by open rate, since that's the number a subject line seems to control. The problem is that opens are measured by a tiny hidden image that loads when the message is displayed. Many mail apps now load images automatically in advance, which can record an "open" before a person has even seen the message. Other readers block images, which hides real opens. So the open figure drifts away from what really happened.
Replies are harder to fake and are the thing you want anyway. A subject line that gets 60 percent opens and no replies has failed. A subject line with an unremarkable open count and four replies has worked. More on this in why to stop measuring opens.
If you turn tracking pixels off, which is a good idea for small-volume cold email, you won't have an open rate at all, and you'll be forced to judge subject lines the way you should: by the answers you get.
Rocketship's AI worker writes a subject for each email based on what it knows about the person, keeps it short, and replies on the same thread when it follows up. It sends from your own Gmail, so the message arrives from a name your buyer can look up. You can read how that works on the AI workers page, and the cold-email basics in how to send cold email that is not spam.
A good subject line gets the email read, but it's the follow-up that gets the answer. Rocketship finds the buyers, emails them from your own Gmail, follows up, and books the meeting on its own. Plans that get you customers start at $24.99 a month (Launch), and every monthly plan starts with a 7-day free trial, card required.