Guide · Email
Email Subject Lines: Simple Tests That Lift Open Rates
Email subject lines are the first thing a reader sees, and often the only thing. Write each one to say plainly what the email holds, follow the sending rules, and test one change at a time. Treat open rates with care, because Apple Mail Privacy Protection now inflates them.
What good email subject lines do
A subject line has one job. It tells the reader what is inside so they can decide to open it. A clear line beats a clever one, because a reader who feels tricked will not open the next email either.
Keep the line short. Many phones cut off long lines after about 30 to 40 characters, so put the key words first. Name the real topic, the real offer or the real date. Write in plain words, the way you would speak to one customer.
Avoid shouting. All capital letters, long strings of exclamation marks and vague teasers make a message look like junk. Say what the email is, and say it once.
Rules your subject line must follow
The law sets a floor. In the United States, the FTC's CAN-SPAM guide says a subject line must accurately reflect the content of the message. A line that promises a discount the email does not contain breaks that rule.
Agencies that send to Singapore readers should also know the Spam Control Act. A checklist from the Direct Marketing Association of Singapore, hosted by the regulator IMDA, says the subject line must be honest, clear, legal and decent. It also says unsolicited bulk commercial email must start the subject with the six characters <ADV> followed by a space, and must carry a working unsubscribe link that is honoured within 10 working days.
That checklist is dated 2007, so read the Act itself or ask a lawyer before you rely on it for a live campaign.
Sources: US FTC, CAN-SPAM Act compliance guide for business; DMAS, Email Marketing Checklist (hosted by IMDA)
How to write five different lines
Start with one email and write several lines for it. Each should take a different angle:
- Plain topic: "June results for your ad account".
- Benefit: "Cut your reporting time with one new dashboard".
- Question: "Is your tracking set up for next quarter?".
- Date or deadline: "Booking closes Friday for the July workshop".
- Name or place: "A note for Singapore retailers".
Only use a deadline that is real. Cut any line that you could not defend in front of the reader. Then pick the two best for a test.
How to run a fair split test
A split test, also called an A/B test, sends two versions to two groups at the same time. Whichever does better wins. Mailchimp's help page says a test can change one of four variables: the subject line, the From name, the content or the send time.
Follow these steps:
- Change only the subject line. Keep the sender, the body and the send time the same.
- Split the list at random, not by name or sign-up date.
- Send both versions at the same moment.
- Decide the winning measure before you send. Pick one, such as clicks.
- Wait long enough. Most opens and clicks come in the first day or two.
- Write down the result, then test something new next time.
Small lists give noisy results. If each group has only a few hundred people, a gap of a few opens means little. Treat a small win as a hint and repeat the test before you change your habits.
Why open rates are less reliable now
Open rates used to be the usual score for a subject line. Email programs counted an open when a tiny invisible image loaded. That count has become less reliable.
Apple announced Mail Privacy Protection in June 2021 as part of iOS 15. Apple says it stops senders from using invisible pixels to collect information, so they cannot tell when an email is opened, and it masks the reader's IP address. iOS 15 reached the public on 20 September 2021. The feature applies to people who use Apple's Mail app and turn it on.
For a marketer, that means some opens are machine loads, not people. A rise in opens can be fake. Marketing trade coverage at the time said teams would need other ways to judge engagement.
So read opens with care. Compare tests only inside the same list and the same week. Look at clicks, replies, bookings and sales as well. A subject line that gets fewer opens but more sales is the better line.
Sources: Apple Newsroom, Apple advances its privacy leadership with iOS 15; Wikipedia, iOS 15; The Current, What Apple's iOS 15 update means for marketers
A simple monthly routine
You do not need a big tool to improve. Pick one regular email, such as a monthly newsletter. Each month, test two subject lines on it and record both in a small table: the date, the two lines, the winning measure and the result.
After six months you will see patterns for your own readers. Maybe they like plain topics. Maybe numbers help. Your own data beats any list of tips, including this one.
When you hire an agency to run email, ask to see its test log. A good one can show what it tested, what it learned and what it changed.
Sources