A strong subject line can improve the open rate of your marketing emails by more than 20 per cent and double the chance they will make it past spam filters and into your subscribers’ inboxes. That’s the finding of our recent research testing different subject lines for clients in the interests of maximising the ROI they receive for their email marketing efforts.

Over a six-month period we ran a series of A/B tests sending two different versions of various newsletters to clients’ mailing lists. In each case, both versions carried the same content, but the “A” version sent to half of the mailing list had one subject line, and the “B” version carried another.

Our top five findings are below. Considering these when writing the subject line for your next newsletter will help you get the most out of the hours you commit to building your mailing lists, writing quality content and sourcing strong imagery and graphics.

Note: Company and event names have been removed from the subject lines mentioned below to protect client privacy.

The first few words of your subject line matter

Emails carrying subject lines with key information or a strong call to action in the first few words are opened by more people. In our tests, an email with the subject line “Book a free consultation with [COMPANY NAME]” was opened by 21 per cent more people than the same marketing email carrying the subject line “[COMPANY NAME] free consultations at [NAME OF EVENT]”.

Key learning: People receiving your emails will often see only the first four or five words of your subject line when it appears in their inbox – the rest may be cut off. The number of words they see depends on factors like their email client and screen size, but the principal remains the same – make sure your first few words grab attention.

Include a grabbing preheader

Many browsers will show a ‘pre-header’ directly under your subject line. In our tests, emails that used pre-headers saw a noticeable increase in open rates.

Key learning: Including a pre-header provides another opportunity to grab your readers’ attention and explain what your newsletter is about. It’s worth using it to engage your reader and provide additional information that isn’t in your subject line. If you’re sending a monthly newsletter, you could use the pre-header space to highlight the lead item or some of the most exciting content.

Dashes in subject lines can be bad news

Our tests pitted subject lines such as “August webinars – [COMPANY NAME]” against variants without any punctuation, such as “[COMPANY NAME] August webinars”. We found that emails with subject lines that didn’t include dashes were opened by seven per cent more recipients than those dashes. They also bounced 130 per cent less, on average, than those with dashes.

Key learning: The finding that putting dashes into subject lines negatively affected open and bounce rates was one of the more surprising findings of our testing. This may be because emails with dashes are less conversational and therefore appear less “natural” to both readers and spam filters. If anyone can fully explain this unexpected result, we would love to hear from you.

Certain words in subject lines will trigger spam filters

Spam filters search emails for particular trigger words and phrases, and one of the first places they look is an email’s subject line. In our tests, we found that emails that included the words “free” or “special” in the subject line bounced 57 per cent more than those that did not. However, this did not occur for our tests distributing email marketing messages to smaller mailing lists – see the finding below.

Key learning: Many of the trigger words spam filters search for are those commonly found in email marketing messages. Consider the words you use in your subject line, and try to avoid overusing spam trigger words. For a more comprehensive list of common spam trigger words, read this HubSpot blog.

The size of your distribution list matters

In our tests, marketing emails sent to a mailing list of a few thousand saw a significant increase in bounces if trigger words were used. However, marketing emails using those same trigger words sent to mailing lists of just one or two hundred saw no noticeable difference in bounce rates.

Key learning: Our tests suggest that if you are sending your marketing emails to a smaller distribution list, spam filters may not pay as much attention to the use of trigger words in your subject line. That’s not free license to use them – it’s safest not to – but it does seem that if your mailing list is large, the level of scrutiny your email comes under from spam filters appears to be higher.

The findings above are the result of real-world testing we’ve conducted over the past few months, but they shouldn’t be read as absolute gospel. Perhaps the most important finding we’ve come away with is that it pays to experiment to find out what works best for your audience.

If you’ve got any thoughts on any of the findings above, or any insights to add, feel free to leave a comment below.