There are two aspects to messages transmission – one from the sender and from the receiver. Once the sender has determined what message to send, the message is signed (using the sender’s private key), encrypted (using the receiver’s certificate), and the message integrity is calculated using a hash. This signed and encrypted message is transmitted over the wire to the receiver. Once the message is received, it is decrypted (using the receiver’s private key), validated (using the sender’s public key), processed and a signed Message Disposition Notifications (MDN), if requested, is sent back to the sender to acknowledge successful delivery of the message. Refer to the documentation on how AS2 handles message transmission.