How to write a cold email subject line that gets opened

Your cold email can be perfectly written — great hook, relevant value prop, clean CTA — and still never get read. Because if the subject line doesn't earn the open, nothing else matters. The subject line is the door. Everything else is the room.

Here's what actually works, based on what consistently gets higher open rates in B2B outbound — and what to stop doing immediately.

The one job a subject line has

A subject line has exactly one job: make the recipient curious enough to open. It is not a pitch. It is not a summary of the email. It is not a place to prove you did your research.

Every decision you make about the subject line should be tested against that single question: does this make them want to open the email? Not "is this accurate?" Not "is this clever?" Does it make them open.

What works

Short and specific. Subject lines under 50 characters consistently outperform longer ones on mobile. The inbox preview cuts off at around 7–10 words. Get to the point before it does.

Name or company in the subject. Personalisation at the subject line level still works — but only when it's natural. "Quick question, [Company]" still outperforms "Quick question" in most tests. "[Company] — outbound" is low effort but effective if the body delivers.

Curiosity without clickbait. Subject lines that create an information gap — something the reader wants to know the answer to — get opened. "Why your reps aren't booking from the list" makes the reader want to know. But the email has to pay it off. Clickbait that misleads kills reply rates.

Directness. "Triage for [Company]" is bland but honest. It gets opened by people who are at least mildly curious and not irritated. Honest and direct often outperforms clever in B2B.

What doesn't work

These patterns are so widespread that buyers' brains now skip them entirely. The "Re:" trick — making it look like a reply thread — used to work. It's now so overused it actively signals cold email, which is the opposite of the intended effect.

The formula that holds up

For most B2B cold email, the subject lines that perform consistently share the same structure: specific context + relevant implication. Sometimes as a question. Sometimes not.

None of these are revolutionary. They work because they're specific, short, and create a reason to open without overpromising.

Test, don't guess

The rep who's been using the same subject line for six months and wondering why open rates are low is doing a version of the same mistake every week. The best outbound teams treat subject lines like split tests — two variants per sequence, track what opens, iterate.

You don't need a big sample. Fifty sends is enough to see a pattern. If variant A is getting 35% opens and variant B is getting 18%, that's signal. Use it.

Most cold emails don't fail in the body. They fail before the body. A subject line that earns the open is the single highest-leverage edit you can make to your outbound. Write one worth opening.