23 email subject line formulas that actually get opened (with data)

We analysed open-rate data across 4 million emails sent through SES Mailbox over the last 12 months. Here's what the numbers actually show — and the formulas behind the subject lines that consistently beat the average.

The baseline: what average looks like

Across all industries in our dataset, the average open rate was 28.4%. B2B lists skew higher (32–38%). E-commerce skews lower (18–24%). Newsletter lists depend heavily on how engaged the list is — fresh lists with double opt-in averaged 41%.

If your open rate is below 20%, the subject line is rarely the only problem. List hygiene and sender reputation matter more. But above 20%, subject line optimisation can meaningfully move the needle.

23 formulas that work

Curiosity gap

Tease the answer without giving it. Works because the brain hates unresolved questions.

• "Why your emails land in spam (it's not what you think)"
• "The one Mailchimp setting nobody turns off"
• "I made a mistake with our last campaign"

Specific numbers

Numbers in subject lines outperform vague superlatives by 17% in our dataset. Specificity signals credibility.

• "We saved $13,800 switching from Mailchimp (the receipts)"
• "3 subject line mistakes killing your open rate"
• "Your campaign reached 94.2% of subscribers. Here's the 5.8%."

Direct question

Questions create an implicit commitment to answer. Keep them short and personally relevant.

• "Still paying per contact?"
• "When did you last clean your list?"
• "How much is your ESP actually costing you?"

The "re:" trick

Looks like a reply in the inbox. Use sparingly — overuse trains your audience to ignore it.

• "re: your account"
• "re: the email I promised"

First-name personalisation

In our data, first-name personalisation adds 2–4 percentage points in open rate on average — but only on lists where the name field is clean. Broken personalisation ("Hi ,") has the opposite effect.

• "{{first_name}}, your monthly send report is ready"
• "Quick question for you, {{first_name}}"

Urgency (without crying wolf)

Time pressure works when it's real. If you manufacture fake deadlines consistently, your list stops believing you.

• "Rate increase in 48 hours"
• "Last day to export before we retire this feature"

Social proof

• "What 4,200 newsletter operators switched to instead of Mailchimp"
• "The approach 80% of SES senders get wrong"

The blunt statement

No fluff. Just the point. Works especially well for technical audiences.

• "AWS SES production access: here's the exact wording that works"
• "Your unsubscribe link is probably broken"
• "Mailchimp raised prices again"

What does not work

• Emoji spam: 1 emoji, strategically placed, can lift opens. 4+ emoji tank deliverability scores.
• ALL CAPS: triggers spam filters and reads as shouting.
• Generic newsletter names: "[Company] Monthly Update" is the subject line of a list people mentally unsubscribe from but never actually do.
• Vague teasers: "You won't believe this..." — worked in 2014. Now it reads as spam bait.
• Long subject lines: Over 60 characters gets cut off on mobile. Under 45 characters is ideal.

The only reliable way to know what works for your list

None of the above is a substitute for your own data. Send two variations to 20% of your list, wait 4 hours, and send the winner to the remaining 80%. Do this consistently and you will accumulate a personal playbook that beats any generic list.