You might ask, which subject line converts better? which template content? which sender?...
The answer to all these questions is A/B split testing.. You need to test to know what works and what does not.
To start go to campaigns > manage A/B split campaigns and create new.
In this A/B test there are 3 options you can test:
Subject line
template or content
Sender name/email
You can choose one of the options above and follow the instructions.
In the last step you can choose the percentage of each variation or keep it by default. You can also define the period of time to start the A/B test.
Once A/B test is fully completed, you can check the results from report and then you will see which variation is the winner.