March 6, 2026·6min read

What Makes a Cold Email Subject Line Work (With Real Examples)

What separates cold email subject lines that get opened from ones archived: ideal length, real personalisation, curiosity gaps, and what to avoid.

Cold EmailCopywritingB2B Sales

Your prospect decides whether to open your email in about half a second, and they make that call on two things only: who it appears to be from, and the subject line. The body can be a masterpiece, but if the subject line does not earn the open, nobody reads a word of it. That makes the subject line the highest-leverage piece of copy in any cold outreach campaign, and the one most people write last and least carefully.

This post breaks down what separates subject lines that get opened from ones that get archived, drawing on patterns from high-performing B2B cold emails across SaaS, recruiting, agencies, and professional services: length, personalisation, the curiosity gap, and the mistakes that quietly tank open rates.

Open rate is a vanity metric until it is not

Open rate is easy to obsess over and easy to game. A vague, clickbait line lifts opens while crushing replies, because people feel tricked the moment they read the body. The goal is not the highest open rate. It is the highest open rate among people who could plausibly become customers. So judge every subject line on two axes: does it get the open from the right person, and does it set up the body honestly so the open converts to a read and then a reply? A line that wins on opens but loses on replies is a trap.

Length: shorter wins, especially on mobile

More than half of business email is now opened on a phone, and mobile clients truncate subject lines hard. On a typical phone, you get roughly 30 to 40 characters before the line cuts off. The practical rule:

Aim for 3 to 7 words. This is the sweet spot in most cold-email tests.

Front-load the hook. Put the most specific word first, in case the rest gets cut.

Cut filler. Words like "quick," "just," "regarding," and "following up" add length and signal a sales email.

Real examples, trimmed

Weak: "Following up regarding a potential partnership opportunity for your team"

Better: "Partnership idea for [Company]"

Weak: "I wanted to reach out about your hiring process"

Better: "Your Q3 engineering roles"

The better versions read like an internal note, not a pitch.

Personalisation that actually means something

"Personalisation" has been watered down to mean dropping a first name into a template. Recipients have seen `Hi {FirstName}` ten thousand times, and a merge tag now reads as automated, not personal. Real personalisation references something only that person, company, or moment would trigger.

Tiers of personalisation, weakest to strongest:

Name only: "John, quick question." Low signal, barely better than nothing.

Company-level: "Idea for [Company]'s onboarding flow." Shows you know who they are.

Role or activity-level: "Your post on RevOps hiring." Shows you did homework.

Trigger-based: tied to a job posting, funding round, or new hire. The strongest, because it implies relevance and timing in one line.

The strongest cold subject lines reference a real, current detail. In recruiter outreach, a subject like "Your open backend role" lands because it points at something the recipient is actively working on. This is where targeted prospecting pays off: with Annabot you can search LinkedIn profiles by country and seniority, or use recruiter search mode to surface people tied to live job postings, which gives you a concrete hook instead of a generic guess.

A personalised line only works if the body delivers on it. Reference the funding round in the subject, then pitch something unrelated, and you have spent your credibility.

The curiosity gap, used responsibly

A curiosity gap is the space between what the reader knows and what they want to know. A good subject line opens a small, specific gap that the body closes. A bad one opens a vague gap the body never closes, which feels like bait and switch.

Patterns that create honest curiosity:

The unfinished thought: "Two things about your pricing page"

The specific question: "Is [Company] still handling X manually?"

The mild pattern interrupt: "Probably not a fit, but" (unexpected and low-pressure)

The number with no context: "3 competitors doing X" (you open to learn who)

What kills curiosity: over-promising ("This will change how you sell"), fake urgency ("Last chance" with no deadline), and irrelevant mystery ("You won't believe this"), which is clickbait that erodes trust on open.

The test is simple. After someone opens, do they feel the subject line told the truth? If yes, you built curiosity. If no, resentment.

What to avoid: the deliverability and trust killers

Some choices do not just lower opens, they push your email toward the spam folder or get you marked as a sender to ignore. Avoid:

Spam-trigger phrasing: "Free," "guarantee," "act now," "100% off," excessive punctuation, and ALL CAPS. Filters weight these heavily.

"Re:" or "Fwd:" fakery. Faking a reply to a thread that never happened bumps opens once, then destroys credibility and invites spam complaints.

Emoji overload. One emoji can occasionally help in some markets; three look like marketing spam.

Misleading specifics. "Per our conversation" when there was none is the fastest way to get blocked.

Generic mass-blast tone. Anything that obviously went to 5,000 people gets treated that way.

Deliverability also depends on factors beyond the subject line, mainly domain reputation and authentication. Sending from your own domain over your own SMTP or a provider like Resend, rather than a shared blast tool, gives you more control over that reputation. The subject line is the visible layer; sender health is the foundation.

A simple framework you can apply today

When you sit down to write a subject line, run it through this checklist:

Is it under 7 words? If not, cut.

Would it survive truncation at 35 characters? Front-load the hook.

Is there a real, specific detail? Not a merge tag, an actual fact about them.

Does it open an honest curiosity gap the body will close?

Does it avoid every spam and trust trigger above?

Does it read like a note from a colleague, not a campaign?

Then test. Write two or three variants per campaign and compare opens and replies on a meaningful sample. Performance is audience-specific, so what works for a CFO will not work for a junior recruiter.

Next steps

Start with one campaign. Rewrite your current subject line into three versions: one company-level, one trigger-based, one curiosity-driven. Keep them under seven words, front-load the specific word, and make sure each is something the body can honestly deliver. Track replies, not just opens. Strong subject lines are not clever. They are specific, short, and true.