How can you setup a post-purchase email trigger?

There are two types of emails - one-time blasts and ongoing email triggers.  Ongoing email triggers send on an ongoing basis.

In order to set one up, first click the + icon, and that will land you in the email template gallery:

For example, you can click on the Review Request, which is one of the only email trigger templates available at the moment.

That will clone that email into your account:

If you'd like to tweak the segment / timing of the email trigger at all, you can do that using the segment builder tool:

In this case, the default is to send 10 days out from the customer's order.  Once you're done, you can publish the trigger as you would normally with any message.

And in your message view, you'll notice that the email triggers have a lighting flash icon next to them.