Delivery Status Notifications (DSN) are messages generated by a mail server to notify the sender that the email has been delivered to the destination mail server.

Problem is, many systems sending email messages are doing DSN requests, but are automated systems and dont accept email messages. This results in your mail server getting clogged with junk outbound email messages, and/or your mail server gets added to a black list.

A quick way to detect if your MX is handling DSN is to telnet to port 25 on the server then type in

ehlo localhost

and you should see something like

220-mx1.mydomain.com ESMTP Postfix
ehlo localhost
220 mx1.mydomain.com ESMTP Postfix
250-mx1.mydomain.com
250-PIPELINING
250-SIZE 154337280
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250 CHUNKING

The above server supports DSN as you can see the line “250-DSN”

To disable this on a Zimbra 8 server ssh to the server, then

su zimbra
postconf -e "smtpd_discard_ehlo_keywords=silent-discard, dsn"
postfix reload

Now it is disabled

References:
RFC1891
RFC3464