The Per Recipient Delivery Response SMTP Service Extension borrows a concept from LMTP (Local Mail Transfer Protocol). LMTP is used to deliver mails on the receiving server to the mailboxes. It assumes, that the server for the mailboxes cannot send bouces and, e.g. due quota limitations, does accept the message for some recipients of the mail transaction and reject it for others. The communication, which recipients receive and reject the message is done after the message body is transmitted. The specitication is in the doc/ directory.
The disadvantage is, that nearly no software implements PRDR. When PRDR is available, aegee-milter does make use of it.