Composing a message
Messages → Compose picks one component of each kind. You can create a component in place while composing (the usual path) or choose one from the library.
A message is not an email by itself. It becomes an email when a pool holds it and a campaign sends it. Preview any message against a real contact from the message page; it renders through the same engine that sends.
Dependencies
Before deleting a message, open Dependencies: it lists every pool that holds it. Removing a message from a live pool shifts the positions after it.