two problems/questions:

our GWIA (2014 SP2) sends outgoing mails to a postfix relay. If postfix
can't connect the target server after a while it generates a "Delayed Mail"-
Message and sends it back to the sending user. This mail does not get
delivered to the user - any idea why?

Second question: if the sending user has delivery confirmation enabled(!)
postfix does not even send a "Delayed Mail" notification.

Both seems strange to me - does this work as designed?