The book's about delivering messages and it's called I Am the Messenger, so it doesn't get much simpler than that. Or does it? Here's the thing: is the book about Ed delivering messages, or is it about a message being delivered to Ed? Egads—things just got real.
When Ed receives the first card in the mail, he checks it out more out of curiosity than anything else. He's surprised when he finds a woman being raped, a lonely lady in need of a friend, and a girl who wants to run and make her dad proud, and we become interested in the ways Ed decides how to handle each situation. There's no guidebook for what to do when you receive a card in the mail with addresses on it, yet Ed takes it upon himself to fix (some of) these people's problems.
And before long, he's solving problems left and right. But what is the message he's actually delivering? It's not the same for each person he comes across. We might say he's just showing a little compassion to people, or caring about them when no one else does—that certainly fits for a lot of the people he meets—but we can't help but notice that his journey is more about his own growth than it is about these other folks and their lives.
All of this makes us think that the title doesn't refer to Ed delivering messages, but instead to him getting them himself. He's learning to care—about others, yes, but even more so about his own life—and that's a pretty powerful message.