What makes a good subject line?
Your subject line, together with your sender name, is the only thing most contacts see before deciding whether to open your email. It is worth spending a few extra minutes getting it right. Here is what works, what to avoid, and how to test your way to better results over time.
Approaches that work
Start with an action word
Opening with a verb creates forward momentum and makes the email feel useful. Examples: "Discover our new collection", "Download your free guide", "Register for our webinar".
Use personalisation
Adding a contact's name or location to the subject line makes the email feel immediately relevant. Examples: "Sophie, your results are ready", "Top events in Antwerp this week". Use the #firstname# placeholder in Flexmail for name personalisation, or a custom field for location. See "Personalising your subject line" for full guidance.
Reference something specific to the contact
For existing customers, mentioning their specific product, account, or purchase history creates strong recognition. "Your subscription renews next week" or "New accessories for your [car model]" work because they reference something the contact already cares about.
Create genuine urgency
Time-based framing increases open rates when it reflects something real: "Today only", "Last 3 spots", "Offer ends Friday". Manufactured urgency that does not reflect reality erodes trust over time.
Ask a question
A well-framed question makes the reader want the answer. "Which marketing channel has the highest ROI?" is more compelling than "Read our new report". The question should hint at value, not just create curiosity for its own sake.
Use specific numbers
Specific numbers outperform vague claims. "5 ways to improve your open rate" is more compelling than "Tips for better email marketing". Write numbers as digits, not words.
Things to avoid
Overly promotional language
Subject lines like "Buy now!", "FREE!!!", or "Limited time offer, act immediately!" trigger spam filters and put readers off. Promotional content is fine; aggressive or all-caps execution is not.
A question followed immediately by an exclamation mark
Some mail clients flag this pattern and treat the message as potential spam. "Want to save money? Click here!" is a combination to avoid.
Misleading subject lines
If your subject line promises something the email does not deliver, you will get opens in the short term but also unsubscribes, spam complaints, and lasting trust damage. Relevance matters more than cleverness.
The preheader: do not forget it
The preheader is the short preview text visible in most email clients right after the subject line, before a contact opens the email. It is valuable space that many senders waste.
Treat your preheader as an extension of your subject line, not a repeat of it. If you do not set it deliberately, most email clients will pull the first readable text from your message, which is often something like "View this email in your browser".
See "Writing a preheader" for full guidance on length, examples, and how to set it in Flexmail.
Testing your subject lines
Flexmail's AB testing feature lets you test two subject lines head-to-head. The variant with the higher open rate wins and gets sent to the rest of your list automatically. Even small, consistent improvements to your open rate compound meaningfully over time.
Support tip Keep a running log of your AB test results. Patterns emerge over time: your audience may consistently prefer shorter subject lines, action-oriented language, or personalisation. That knowledge is more useful than any general best-practice guide.
Common mistakes to avoid
- Making it too long. Most email clients show between 40 and 60 characters in the inbox view. Put the most important information first.
- Using the same subject line format for every campaign. Contacts stop noticing patterns that appear in every email from you. Vary your approach.
- Ignoring the preheader. A blank or auto-generated preheader wastes a significant piece of inbox real estate. Set it deliberately every time.
- Promising something the email does not deliver. The open is only valuable if the contact does something after opening. Subject line and content should align.
Next steps
- Set up AB testing on your next campaign to start testing subject line approaches systematically.
- See "Personalising your subject line" for guidance on using contact data to make subject lines more relevant.
- See "Writing a preheader" to make the most of the preview text that appears alongside your subject line.
- Try the AI subject line and preheader generator if you want suggestions based on your email content. See "Using the AI subject line and preheader generator".