Disable the sending of "Undeliverable Mail Returned to Sender" in Postfix

Found the answer here.

How do I configure Postfix to never send NDR responses? For example, if someone tries to send an email to a user that doesn't exist, I don't want my server to respond and let them know that it doesn't exist. I just simply don't want it to respond.

It is strongly suggested that you do not disable NDR responses on any mail server. However, it is possible to disable NDR responses in Postfix if you would like to.

Open your Postfix master.cf file and find the following line:

bounce unix - - n - 0 bounce

Change it to this:

bounce unix - - n - 0 discard

Next, restart Postfix.

Please be advised that you will see an error in your logs whenever Postfix runs the discard handler instead of the bounce handler. The errors will look similar to this:

Dec 15 16:07:40 websrv1 postfix/discard[15220]: warning: unexpected attribute nrequest from bounce socket (expecting: flags)

Dec 15 16:07:40 websrv1 postfix/discard[15220]: warning: >deliver_request_get: error receiving common attributes