We write every cold email subject line to look like something a coworker would send. A subject that reads like marketing gets deleted before anyone opens the body. So the first test we run is simple: would this look normal in a Slack message from a colleague?
That one test rules out most of what people call clever. It also explains why our subjects are short, plain and a little boring on purpose.
How do we write a cold email subject line?
We pick one of two approaches for each campaign, and every variant in that campaign uses the same one. The first is a subject that blends in, and it's our default. Subjects like customer request or audit prep look like the rest of a work inbox.
The second is a subject that raises a question only the body can answer, so the reader stops on it. We only use it when the email has a clear point of view behind it. Without one, a curious subject is clickbait.
What rules do we follow?
Our mechanical rules are short, and we check every variant against them one at a time.
- We keep a subject to four words or fewer, all lowercase, with no punctuation.
- We keep it under 45 characters, so it shows in full on a phone.
- We use at most one spam word in a subject and never a dollar sign or all caps.
- We never put a fake Re: or Fwd: on a first email, because it deceives the reader.
- We make sure the subject connects to the body, so the email never baits and then switches.
Every subject also ships as a block of three to six full-subject variations, with four or more preferred. A single fixed subject sent at volume becomes a pattern that filters can learn, so we spread the send across several versions. Each variation has to pass every rule on its own.
Follow-up emails work differently from the first. Emails two and four stay in the same thread and never get a new subject, while email three starts a fresh thread with a completely new one. Some subjects include a variable like a company name. We check how each one reads when the data is missing, and we use a fixed subject if the fallback reads badly.
What do we avoid in subject lines?
We avoid anything that sounds like a slogan or a riddle. In one audit of our live subjects, a phrase like team of one got marked down as cringe. A plain, concrete subject tied to the reader's own work passed, even when it was phrased as a question.
We also avoid title-case webinar subjects that read like a press release. We judge a subject by whether it reads like a colleague wrote it about something concrete. A question mark or a first name isn't a problem on its own, so we allow a question when it's concrete.
What do weak and better subject lines look like?
These examples are made up for an imaginary invoicing software company. They are only here to show the rules at work, and the weak versions break at least one rule each.
| Weak subject | Why it fails | Better subject |
|---|---|---|
| Revolutionize Your Billing Workflow Today! | Title case, hype and an exclamation mark | month end close |
| invoices on autopilot | A slogan, not something a colleague would write | invoice approvals |
| Re: our call | A fake reply on a first email | audit prep |
| FREE billing audit for your team | All caps, a spam word and sounds like marketing | acme billing audit |
| Billing Insights Webinar: Faster Close for Acme | Title-case webinar style and over four words | invoice owners |
| a team of one | An abstract phrase that reads like a riddle | late payment reminders |
Each better subject is lowercase, short and tied to something the email body then talks about. The one with the company name (Acme here) only ships if the name reads cleanly, and invoice owners names a concrete role rather than a tagline. Each of these would pass the colleague test.
month end close invoice approvals audit prep acme billing audit invoice owners late payment reminders
How do we test a subject line?
We test two subjects at a time and wait for at least 100 sends on each version. We control for the segment so the comparison is fair.
We measure opens and replies together. A subject that earns opens but no replies is clickbait, so it doesn't count as a win. Our copy checker grades a subject against rules like these.
Common questions
How long is a good cold email subject line?
We keep a subject to four words or fewer and under 45 characters, so it shows in full on a phone. We also write it in lowercase with no punctuation.
Is it okay to use Re: in a cold email subject line?
We never put a fake Re: or Fwd: on a first email, because it deceives the reader. Emails two and four in our sequences stay in the same thread and never get a new subject.
How many subject lines does one campaign use?
Every subject ships as a block of three to six variations, and we prefer four or more. A single fixed subject sent at volume becomes a pattern that filters can learn.
How do we know if a subject line worked?
We test two subjects at a time and wait for at least 100 sends on each version. We measure opens and replies together, because a subject that earns opens but no replies is clickbait.