Set it up as follows:
- Name, description, time zone. etc. These are completely up to you. Keep it disabled until you are ready to go.
- Frequency: set it to run every day, so that MailUp checks for the condition every day, and check the Only run once
- Event triggerCondition. Select After N days from ...
- Enter: 1 as the number of days
- Select "email pendingEmail subscription requested (not yet confirmed)" from the drop-down
- Action: enter a sender email and name, and then select the message you created above as the email to be sent.
Use the simulation feature to simulate what would happen if the trigger were run at that moment: it should give you a number of recipients that equals those that signed up a day before, without confirming their subscription.
Test this with your own "seed list" (a number of different email addresses at different ISPs) or work with our Deliverability Team, if you subscribed to the Delivery+ service.