Currently we support both synchronous and asynchronous MDN responses. This enables you to respond to your trading partners with either a synchronous or asynchronous MDN after receiving an AS2 message. Since synchronous MDNs are sent over the same connection channel as the message, it is much simpler and hence the recommended option. If you need more time to process the message before sending an MDN, asynchronous MDNs are preferred. If you have a need to request and receive asynchronous MDNs when sending messages to your trading partners, contact us through AWS Support or your account manager.