During the SMTP connection between the sender’s mail server and the TMEMS mail servers, Inbound Protection -> Recipient Filter sends an SMTP Error back to the sender’s mail server instead of an NDR. It is up to the sender’s mail server to send an NDR back to the sender.
Example of an SMTP error:
Apr 18 21:55:22 in01 postfix/smtpd[52897]: NOQUEUE: reject: RCPT from unknown[undisclosed IP Address]: 554 5.7.1 <undisclosed recipient email address>: Recipient address rejected: Access denied; from=< undisclosed sender’s email address > to=< undisclosed recipient email address > proto=SMTP helo=<p1186-ipbf206oxxxx.xxxxx.domain.com >