Peppol is a semantic network based on Open Standards. It is a 4 corner model, meaning that a sender and receiver can exchange documents without having the same service provider. There are more than 300 Peppol providers to choose from all over the world. This is known as a 4 corner model as shown below:
The receiver of the documents needs to be registered in a SMP, which works like a DNS and is exchanging an endpoint ID with an URL. The exchange between corner 2 and 3 is standardized in the AS4 protocol and each of these corners need have a Peppol certificate to exchange the documents. The documents are validated before sending and can also be validated when received.