Subject Lines: The Only Job Is to Get It Opened

Spending 90% of your effort on the email body and 30 seconds on the subject line is one of the most common mistakes in cold outreach

A subject line has one job: make the prospect open the email.

Not sell them. Not impress them. Not tell them what the email is about. Just get it opened.

Everything that matters about your email, the signal, the bridge, and the ask, cannot do its job if the subject line fails.

What Actually Gets Cold Emails Opened

Specificity beats cleverness. A subject line that names something about the prospect outperforms a clever subject line every time.

Curiosity gaps beat value statements. "booking flow" gets more opens than "Improve your website conversion rate." The first creates a question. The second sounds like a pitch.

Lower case beats title case or all caps. Lower case looks like a personal email from someone you know. It does not trigger the instant mental categorisation of "marketing email."

Shorter beats longer. 3 to 7 words is the target and under 40 characters. On mobile anything past that gets cut off.

Subject Line Formats That Work

The observation format names the thing you noticed and reads like an informal note.

"booking flow" or "the manual client tracker" or "your new hire post."

The name format uses their first name or company name only for a highly personalised feel with no extra words.

"sarah" as just their first name, "[company name]" as just the company, or "re: [company name]" to imply a continuation of a conversation.

The direct question is short, direct, and uses minimal punctuation.

"worth a quick chat?" or "quick question" or "is this on your radar?"

The referencing format implies you have done something relevant to them before reaching out.

"saw your post about the team growth" or "checked your site" or "re: the booking form."

Subject Lines That Kill Deliverability and Open Rates

Do not use FREE, EXCLUSIVE, or OPPORTUNITY because they are classic spam trigger words.

Do not use multiple exclamation points because they are a spam signal.

Do not use ALL CAPS because it is aggressive and a spam signal.

Do not use ROI, revenue, or percentage claims because they get filtered by most corporate email systems.

Do not use "Following up" as a subject line because no one opens a cold follow-up subject line.

Do not use misleading or false implies like "RE: our call" because it damages trust when they open it and is often illegal.

The Subject Line Prompt

Using @EMAIL.md — write 5 subject line options for this cold email.

The email is about: [SIGNAL — what you noticed]

Prospect name: [FIRST NAME]
Company: [COMPANY NAME]

Rules:

• Maximum 7 words each
• All lower case preferred with no capitalisation except proper nouns
• No punctuation except for a question mark where it fits naturally
• No mention of ROI, value, revenue, or what I offer
• Formats to try: observation only, their name plus observation, short question,

or just the thing you noticed

Give me 5 options ranked from most to least likely to get opened.

Explain in one word why each should work such as curiosity, specificity,

informal, etc.

Testing Subject Lines

If you are sending to 50 or more prospects in a campaign, split your list and test two subject lines. Send Group A of 25 people subject line 1 and Group B of 25 people subject line 2. After 5 days compare open rates and keep the better performer for the rest of the campaign.

The minimum sample size is 100 opens per variant before you trust the data. With 25 recipients per variant your open rate data is directional and not conclusive but it is better than guessing.

Do not test more than one variable at a time. If you test both the subject line and the email body simultaneously you will not know which change made the difference.

A Note on Open Rates in 2026

Since Apple's Mail Privacy Protection launched in 2021, open rate data has become unreliable. Apple pre-loads emails in the background which registers an "open" even if the person never read the email. On lists where a significant portion of prospects use Apple Mail, which is common in the UK and US, open rates can be inflated by 20 to 40%.

This means you should not optimise purely for open rate. Reply rate is the metric that actually tells you if the email worked. Track positive replies meaning interested prospects separately from total replies which includes unsubscribes and out-of-offices.

Subject Lines for Each Email in a Sequence

Your subject line in follow-up emails should be the same as the first email. This creates a thread so your follow-ups look like a continuation of the same conversation and not a new cold email.

Only change the subject line if you are starting a completely different angle in a later touch.

Adding to EMAIL.md

## Subject Line Variants
Campaign: [CAMPAIGN NAME]
Winning subject line: [YOUR BEST PERFORMER]

Reply rate: [%]

Notes: [WHAT WORKED AND WHY]

Your Action Right Now

Run the subject line prompt for your top 3 emails from the previous lesson. Pick the best option for each. Write it at the top of the email draft.

You now have 3 complete first emails with subject line and body. The next lesson builds the sequence around them.