Hello; Ienabled a recipient filter in my SMTP Virtual Server. When a message get filtered the sender gets a NDR with this text " 550 5.7.1 Requested action not taken: mailbox not available."Thistext does not reflect why email was rejected and I would like to customize it.I already tried with IMF inreject mode and edit this registry entry "CustomRejectResponse" in HKLM/Software/Microsoft/Exchange/ContentFilter location. Nothing happens, It seems that recipient filter is not govern by the IMF at all. Is there a way to customize the text on this NDR event? Thank You in advance.

Which is not possiblesince NDR is defined in RFC compliance documentation and Exchange is standard/universal product so should be followed Internet standard. I think, NDR is hard coded in Exchange.