This is one of the common questions we get when prospects come looking for data replication tools. It's more a question of Integration design patterns than of product implements.
Let's get started with what an ESB is - Enterprise Service Bus. This is an integration design pattern where messages are passed so that one or more Message Listeners can listen and consume the message - store and forward. These messages—like, say, emails—have a header (from and to), a payload (the message), and perhaps attachments. Based on the ESB, there might be some limitation on payload and attachments sizes.