To use the subject line tester, create your message as normal and on the second tab of the editor subject lines & open rate tools, under the subject line text area there is an option to add another subject line
You will then notice another subject line text area appear under the original. You can now enter your second subject line. You can enter up to 5 subject lines.
Subject line tester
To perform a classic A/B test on multiple subject lines all you need to do is set 2 or more subject lines and do not tick the ‘Subject line selector’ box. When you send this message in a campaign, all of your subject lines will be split tested across all of the list and the report will tell you their open rates.
Subject line selector
If you would like to have the best subject line automatically chosen for you during the campaign, select the option Yes, I want you to pick the best subject line for me to enable “Subject line selector”.
Subject line selector will use between 5,000 and 10,000 records over 1-2 hours to A/B test the subject lines before sending the winner to the rest of the records.