sendmail doesn't send out non-delivery notices

[Carton Lao]

when an email is sent to this server with an invalid account,the log file showed that sendmail recognized that, and logged "user unknown". But the non-delivery-notice that should be sent to the sender was never sent.

where can be wrong?

this system actually uses SMTP-Relay of OpenMail to listen to port 25, any address not recognized by OM is relayed to sendmail. At least it is supposed to, and the sendmail log file showed that it did.

I have a test system and set up the same way and it works.

[U.SivaKumar_2]

Hi,
Ensure that you are using ESMTP between the servers.
You have configure support for DSN ( Delivery Status Notification ) in sendmail.

Since sendmail 8.10, you can use

define(`confRRT_IMPLIES_DSN', `True')

in sendmail.mc file.

regards,
U.SivaKumar

[Jordan Bean]

Sendmail doesn't return NDRs while receiving messages; It is the responsibility of the remote mail server to return an NDR to the sender. On the other end, Sendmail will attempt to return NDRs when sending fails.

[Carton Lao]

I should know that ndn was sent by the next incoming hop. That gave me confidence that it is NOT the email server configuration. It turns out that the firewall was configured differently and was blocking the sending out of the ndn. We had to put a sniffer on to see the traffic to diagnose the problem, and subsequently fixed it.

Thanks all.