> I am seeing the subjected error for a small percentage of messages, and > then those message stay in the deferred queue. > > from the log: > postfix/local: warning: unexpected protocol delivery_request_protocol > from private/bounce socket (expected: delivery_status_protocol) There is some serverfault webpage that suggests replacing the "bounce" service in master.cf with "discard". While this prevents Postfix from sending bounces, it also causes the discard daemon to log multiple warnings about protocol errors. With Postfix 3.6 and later, this unsupported hack no longer works. Postfix logs an "unexpected protocol delivery_request_protocol" warning, and moves the message to the deferred queue. For Postfix 2.3 and later, here are the offically supported ways to disable bounces. 1) In your mail generating application, append NOTIFY=NEVER to each RCPT TO command. 2) When submitting mail through the Postfix SMTP server, use the smtpd_command_filter feature to inject NOTIFY=NEVER into the RCPT TO command, as described in https://www.postfix.org/postconf.5.html#smtpd_command_filter /etc/postfix/main.cf: smtpd_command_filter = pcre:/etc/postfix/command_filter /etc/postfix/command_filter: # Bounce-never mail sink. /^(RCPT\s+TO:\s*<.*>.*)\s+NOTIFY=\S+(.*)/ $1 NOTIFY=NEVER$2 /^(RCPT\s+TO:.*)/ $1 NOTIFY=NEVER 3) When submitting mail through the Postfix sendmail command line, specify "-N never" as described in https://www.postfix.org/sendmail.1.html -N dsn (default: 'delay, failure') Delivery status notification control. Specify either a comma-separated list with one or more of failure (send notification when delivery fails), delay (send notification when delivery is delayed), or success (send notification when the message is delivered); or specify never (don't send any notifications at all). Hopefully this text will eventually end up in search engines and will be referenced in howtos.