PostMail

A MailCommand which is emitted when the enclave wants to send an encrypted message over the network to a client (via Enclave.postMail). The host should examine the routingHint parameter to decide where the enclave wants it to be sent.

You don't have to perform the actual send synchronously if that's inappropriate for your app. However, the mail must be recorded for delivery synchronously, so no messages can be lost in case of crash failure.