For information interventions to be effective, recipients must first engage with them. We show that engagement with repeated digital information interventions is shaped by subtle and strategically controllable signals of the information's value. In particular, recipients' expectations are shaped by signals from the "envelope" that surrounds a message in an information intervention. The envelope conveys clues about the message but does not reveal the message itself. When people expect the message to be valuable, delivering it in a consistent and recognizable envelope over time increases engagement relative to varying the envelope. Conversely, when people expect the message to be of little value, delivering it in a consistent and recognizable envelope decreases engagement relative to varying the envelope. We show this with two field experiments involving massive open online courses and one online survey experiment (all pre-registered, N = 439,150).