Improving Your Email Marketing ROI With Subject Line Testing

Email Marketing has been around almost as long as the internet, and the reason that it has stood the test of time is purely down to performance. Email continues to deliver a high ROI for the marketing spend required and is relied on by hundreds of thousands of companies around the world to generate leads and sales. There is still a big difference between ‘successful’ email marketing and ‘really successful’ email marketing and that difference boils down to one thing – testing.

The effect that a subject lines length can have on open rate.

The effect that a subject lines length can have on open rate.

There are many things you can test in your email newsletter. One of the easiest to test which can have a big effect on your open rates is the subject line. Most reputable email marketing platforms provide a lot of functionality around subject line testing which can include:

  • Split Testing – Sending your email with one subject line to 50% of your database and a second version with a different subject line to the remaining 50% with accurate reporting to find the best performing subject line for optimising future campaigns.
  • Delayed Testing – A useful feature for companies with large databases. This allows you to divide your database firstly into two 20% segments which receive different versions of the subject line. A set timeframe later (usually one hour) when a winning subject line is decided, the remaining 60% of the database will receive the email with just the winning subject line, maximising results from the campaign.
  • ‘From Name’ Testing – some providers such as Mailchimp allow you to split test the ‘From’ name field which, when combined with subject line testing, will maximise your overall results.


There are some best practice techniques you can follow that will ensure you hit the ground running with your email marketing. The below list of techniques was compiled by Mailchimp who analysed an amazing 200 million emails with open rates ranging from 93% right down to 0.5%. Here is what they found:

  • Straight forward subjects which accurately describe the contents of the email work best.
  • Avoid the following 3 keywords which will negatively effect your open rates: Help, Percent off, and Reminder
  • Use of location helps, providing a city name in the subject helps open rates.
  • The ideal subject length is under 50 characters.
  • The ‘From’ line should remain consistent and communicate either your company or personal name.
  • Subject lines that ask a relevant question can perform better.
  • Repeating subject lines or using similar ones to the same list will have a negative effect on open rates over time.
  • Extremely promotional subject lines will have a negative effect on open rates.
The results of a subject line test involving 3 different variations as detailed by

The results of a subject line test involving 3 different variations as detailed by

Although the above Mailchimp testing insights have given you a headstart, all email marketing databases will respond differently to different subject lines so there is no best practice rules which will guarantee you success. Regular testing and optimisation is the only way you will improve your results over time so what are you waiting for? Here is a few things you can start testing today on your database:

  • The length of subject lines, long vs short.
  • Personalisation of subject lines with subscriber names.
  • Promotional keywords such as Free or Sale.
  • Capatalisation of some or all keywords.
  • The use of symbols such as question/exclamation marks and other icons.
  • The language used, ie. formal, descriptive, friendly, cryptic.


Think you’ve mastered the art of subject line testing? Check out this article which looks at the timing of online marketing and how when you send your email newsletters can have a big effect on your overall results.

Duncan Jones

About The Author - Duncan Jones

I am a growth marketing specialist from New Zealand and im passionate about growing businesses through creative and performance focused digital marketing. I insist on tracking everything, follow proven growth processes and I still love the thrill of getting a first conversion then optimising & scaling the campaigns for clients across a huge range of industries. You can find me on LinkedIn here, find out how to hire me here or you can contact me here.

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