The importance of a strong subject line in any email communication can’t be over stressed. A good one can lead to better open rates and higher conversions while a bad one renders even the best creative worthless.

Here are some guidelines for marketers keen to make the best possible impact.

1. Set up a ‘test and learn’ plan

Marketers that spend time testing will outperform their peers. However, this doesn’t mean you have to test every single message. This is time consuming and will result in more of your customers receiving under-performing subject lines. Instead, marketers should plan ‘test and learn’ cycles into their campaign schedule to determine what general rules work best. Work smarter, not harder.

2. Use themes

The hardest part of subject line testing (and often the least scientific) is the selection of the lines themselves. Brainstorm different themes that you believe will appeal to your customers, such as the following, then test examples of each theme during each ‘test and learn’ cycle.

  • Urgent call to action, e.g. “This weekend only…”
  • Curiosity, e.g. “Which toy will be the run-away success story this Christmas?”
  • Offers, e.g. “Save 20% on product X”
  • Product features/benefits, e.g. “New laptops now in – our fastest ever processor speeds”

3. Consider variation

Once you’ve established which themes work best with different customer segments, consider the number of campaigns you’ll be sending before the next test cycle. Open rates will probably decline if the same themes are used regularly, so evaluate whether there’s a close second or even third option that can be used occasionally? If so, how frequently do you want to use each theme, given the number of campaigns you have planned?

Also, test subthemes such as:

  • Personalisation
  • Number of characters
  • Use of your brand
  • Percentage savings versus £ values in offers

4. Don’t mislead customers

Don’t place your customers’ trust in your brand at risk when optimising open rates. Create subject lines that are representative of the email’s content – otherwise response rates in future campaigns will suffer.

5. Target customers in different countries

Don’t assume that subject lines or themes will perform equally well in different regions. Test emails locally in each country in which you operate. It’s generally better for a native speaker to draft new subject lines from scratch. A turn of phrase that works well in English will often be less effective once translated.

6. Ensure results are statistically significant

When designing a test it’s important to consider how many variants will be assessed. The more options, the larger the total sample size will need to be. Many marketers apply testing on a fixed percentage of their list, say 10%, before mailing the remaining 90% the winning subject line. This approach has the advantage of being easy to apply, but may not yield reliable results if the list size is too small. Do some research to design an appropriate methodology – there are plenty of accessible overviews of significance testing online.

7. Be consistent

Consistency is key in testing. It’s important to send all subject lines out to a cross-section of your customers simultaneously, using the same ‘From’ address and via the same IP address. This will ensure that the only variable measured is the subject line itself.

8. Wait and see

Because campaign timelines are often tight, many marketers send their subject line tests a few hours before deployment. Open rates typically spike one to three days post-deployment. Try to give respondents this long to respond, otherwise you’ll only be assessing which lines appeal to your most engaged customers.

9. Document findings

A common mistake by marketers is failing to keep adequate records of test findings. Dates, subject lines, themes, and campaign metrics should all be documented. It’s shocking how often best practice is forgotten when a marketing manager moves roles. Document all findings in a single location so they are easily comparable.

10. Test, test and test again

Retesting is an important part of the process. Just because personalisation, for example, generates an uplift in an initial round of testing, doesn’t mean the same approach should be used month after month.

Marketers spend much of their time trying to decide which opportunities offer the best returns for their limited resources. Try to make subject line testing one of your priorities; it’s one of the simplest ways to drive ROI growth.

Jon Maddison

Jon Maddison


Jon Maddison is Vice President and General Manager of Epsilon, UK.