Hi Dylan Teo​, It looks like in your screenshot that there's been only 1 Challenger set up. Marketo will not acknowledge the original subject line in the "Champion" email as a contender, and you'd need to list out 1. (Original Subject Line) and 2. (New Subject Line) , 3. (Other New...), etc. When you are using an Email Send (mailbox) program, it is most effective for when you want to set up a consecutive flow for an A/B Test: 1. Run A/B test for a % of the population. So if you select 60%, 30% will receive test A and 30% will receive test B. 2. Sending winner to remaining population, which in this case is 40% - which you can set based on an engagement metric (open, click, score, etc) or to manually determine the winner before the next scheduled date. To test 100% of the population, like others suggested, using a trigger campaign (in any program) would be most optimal.... and there's a few ways I like to do: [Request Campaign trigger] where you have two smart campaigns set up: 1. The A/B Send campaign . (Trigger) - Smart List: Add a Campaign is Requested Trigger, with Marketo Flow Action as Source - Flow: Set up Send Email step with the email that already has the A/B Test approved (like your screenshot). - Schedule: Then click Activate so the trigger is ready to fire! 2. The Email Send Campaign    (Batch) - Smart List: Set up your batch as needed in the smart list. In Flow, set up "Request Campaign" - Flow: Use a flow step, Request Campaign, and then search for the "A/B Send" trigger campaign that you just set up. Remember it needs to be Activated in order to appear in the dropdown. - Schedule: When you are ready to have the A/B test rocket fire away, you can schedule this batch smart campaign. Before you do so, highly recommended to double check your email's A/B test, which you can do by right clicking on the email asset and "Send Sample Test". -------------------------------------------------------- -------------------------------------------------------- [Add to List trigger] by using a smart list, static list, and a trigger campaign. 1. Create a Static List which you will use for the entire A/B test population. 2. The A/B Email Send   Set up just like the first example's , except use a Add to List trigger instead of a Request a Campaign. Then in dropdown, select the Static List you just created. Remember to Activate again! 3. Set the Trigger So there's a couple ways around this.... similar to OR you can create a Smart List to shape the email audience as you please ....then you go to the People tab and "Select All" ....then go to Person Actions > Marketing > Add to List                ....then you select the static list (from step 1) and confirm to run.  This will trigger the A/B test immediately! (Another creative method? use this method with a "Request a Campaign" trigger and then when reviewing a separate Smart List, you can also go to Person Actions > Special > Request a Campaign.  Then, you select the "A/B Send" trigger campaign that you just set up. Once you request this campaign, the trigger will immediately fire and send the A/B test. So again, be prepared with all your check points before doing this.) ... View more