So the email I am waiting for is on my "waiting for list" but how to I record that I need to send out other emails when that email comes in (with the addresses that I need)

One next action of email x and y when email from z comes in? but it isn't an action I can do straight away? Im a little confused (I admit it doesn't take alot )

How do I leave a bookmark that this is what i need to do next?