Email Response Rates, Email Writing Tips

The Ideal Email Subject Line Length

We ran multiple experiments on subject lines and summarized one particular case study example in our recent webinar: “Use Split Testing to Improve Email Responses (you can download the slides and view the on-demand version).

Here are the email subject variations:

1: %%First%% – [Webinar] Split Test Case Studies (Feb 4)

2: %%First%% – [Webinar] Split Test Case Studies (Feb 4) *Note

3: [Webinar] Case Studies Webinar: Using Split Testing to Improve HTML Email Response Rates
(Feb 4th) – Please Join Us

4: [Webinar] Split Test Case Studies (Feb 4)


  • In version #2 we personalized the email introduction and used the recipient’s first name in several parts of the email.
  • In the subject lines that have squarre brackets – ‘[‘ and ‘]’ – we included the brackets as noted.

Here are the results based on the email open rates:

There worst performing version had a 30% open rate; the best performing version had a 42% open rate. What is interesting though is the click-through rates for the same campaigns:

Email Response Rates - Graph

The shorter subject line far outperformed all versions (as much as a 520% improvement vs. the worst performer) and the log subject line far underperformed as compared to the other 3 versions.

Other relevant articles:

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