Email Testing Best Practice : Subject Line Tests

As always, be sure that your A/B list split is clean. Particularly in case of an email subject line test. An imbalance in user states or email service provider can create huge variations in test results and it will get difficult to analyze and study the results. With an uneven balance you're incredibly likely to have invalid open-rate data at the end of the day.

Test email subject line concepts over several sends. Even if you are incredibly cautious about the quality of your A/B split, there are still factors that you won't be able to account for such as long-tail opens and possible timing issues. In general, test your subject line concept three to five times before determining what the data means.

Be aware of inbox deliverability! Your subject line can have the greatest impact on your ability to get into the inbox instead of the spam or junk folder. If you're testing multiple subject line concepts, be sure to always check deliverability before you send to your list.

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