How to Write Email Subject Lines That Get Opened

Your email subject line is the most important few words you will write in any campaign. It is the first thing a subscriber sees in their inbox, and on mobile it is often the only thing they see before deciding whether to open or delete. A brilliant email with a weak subject line will go unread. A mediocre email with a compelling subject line will at least get a chance to do its job.

Writing good subject lines is part craft, part science. There are principles that reliably work, patterns to avoid, and the value of testing to find what resonates with your specific audience. The good news is that improving your subject lines is one of the fastest and easiest ways to lift your email marketing performance without changing anything else.

Principles That Make Subject Lines Work

The most effective subject lines do one of three things: they create curiosity that can only be satisfied by opening the email, they offer a clear and specific benefit, or they signal urgency or relevance. "The mistake most businesses make with their homepage" creates curiosity. "5 ways to reduce your tax bill before April" offers a specific benefit. "Last chance: offer ends tonight" signals urgency. The best subject lines often combine two of these elements.

Specificity consistently outperforms vagueness. "How to improve your website" is weak. "3 changes that doubled our client’s enquiries in 30 days" is strong. Numbers, named outcomes, and time frames all add specificity. They signal that the email contains something concrete and actionable, not generic advice the subscriber has already seen elsewhere.

What to Avoid

Spam trigger words can cause your emails to be filtered before they even reach the inbox. Words and phrases like "free," "click here," "earn money," "limited time offer," and "100% guaranteed" are well-known filters. This does not mean you can never use these words, but overusing them — especially in combination — raises your spam score. Most email marketing platforms will score your subject line before you send; take their warnings seriously.

All-caps subject lines, excessive exclamation marks, and misleading teaser lines also damage trust over time. If a subscriber opens your email expecting one thing and finds another, they are less likely to open the next one. Subject lines should accurately reflect the content of the email — the goal is to attract the right opens, not to trick people into clicking.

Testing and Optimisation

A/B testing subject lines is straightforward on most email platforms. You write two versions, the platform sends each to a portion of your list, and after a set period it sends the winning version to the remainder. Over time, this builds a clear picture of what language and approaches your particular audience responds to — which may be different from what the general research suggests.

Keep a running log of your subject lines and their open rates. After ten or twenty campaigns, patterns will emerge. You may find that questions outperform statements, or that your audience responds better to specific numbers than to general promises. This data is specific to you and is more reliable than any general best-practice article.