A good subject line tells the right reader, quickly and honestly, what is inside and why it matters to them. Preview text then finishes the job. Neither is a trick: the most reliable approach is to write clearly, keep them short enough for a phone, never promise what the email does not deliver, and test one variable at a time on your own list.
What the reader sees first
Mailchimp’s help page on preview text explains that when someone receives your email, the first things they see are the From name, the subject line and the preview text. If you do not set preview text, email clients pull in the first part of the message, which can be unhelpful material such as a “view in browser” link. It recommends keeping preview text short, putting the most important detail first and using it to support the subject rather than repeat it.
So think of three elements working together:
| Element | Job | Example approach |
|---|---|---|
| From name | Trust and recognition | A consistent business or person name |
| Subject line | Topic and relevance | What it is and why it matters |
| Preview text | Extra context or reason to open | Expands on the subject, does not repeat it |
Principles for subject lines
- Be specific. “Three checks before you book a website redesign” beats “Newsletter, issue 14”.
- Lead with the useful part. Many inboxes cut off long lines, especially on phones.
- Keep it short. Mailchimp’s guidance on subject lines suggests no more than 9 words and 60 characters, particularly for mobile readers. Treat that as a guide to test, not a law.
- Match the promise to the content. A misleading subject line may win an open and lose trust, and it can attract spam complaints.
- Use personalisation only when it helps. Mailchimp says merge tags such as name or location can raise opens for most users, particularly with targeted automations like a birthday offer. Overuse can feel intrusive, so use data you hold properly.
- Go easy on punctuation, capitals and symbols. Mailchimp advises keeping punctuation to a small number of marks, and using at most one emoji, alongside words rather than in place of them, since emojis can render differently across operating systems.
What to test
Test one variable at a time so you know what caused a difference:
- length (short versus descriptive)
- a question versus a statement
- a benefit versus a curiosity gap
- personalisation versus none
- a number versus no number
- tone: plain versus playful
- preview text: supporting versus none set
Mailchimp describes A/B tests that differ only in the subject line, sent to random portions of your audience with the best performer sent to the rest, and notes this shows what your own contacts prefer rather than what a generic benchmark suggests. Features and plan limits vary between platforms, so check yours.
Running a fair test
- Write a hypothesis: “A specific benefit in the subject will beat a vague teaser.”
- Change only that element.
- Split the list randomly.
- Decide the success metric first. Opens are easy to measure but an imperfect signal, so prefer clicks or enquiries where you can.
- Wait long enough for most responses to arrive before choosing a winner.
- Record the result and apply it, then test something new.
Small lists produce noisy results, so treat a single test cautiously and look for patterns over several sends.
Respect the rules
Subject lines are part of a commercial message, so the general rules still apply. The ACMA’s spam guidance requires consent, accurate sender identification and an easy unsubscribe. Avoid pressure tactics that are untrue, such as fake deadlines. Claims in a subject line need to be as accurate as claims anywhere else; see advertising law basics.
Writing resources
Subject lines are short ad copy. Frameworks in our ad copywriting guide and the habits in ad creative best practices apply directly, and where the email sits in the buyer journey is covered in the marketing funnel explained.
A quick checklist
- One clear idea per subject line
- Key words in the first 30 or so characters
- Preview text written deliberately
- Promise matches the content
- Testing one variable at a time
- Results recorded
Limits
Open rates are an imperfect measure, and results from other companies’ lists may not transfer to yours. Your own data, treated carefully, is the best guide.
Where Radiance Impact fits
Radiance Impact’s Growth team supports campaign planning, copy and measurement. See Growth or contact us if you want help building an email programme.
Sources
- Best practices for email subject lines, Mailchimp
- About preview text, Mailchimp
- Avoid sending spam, Australian Communications and Media Authority
