On 28/01/18 16:28, mick wrote: [snip]
Your response actually confirmed that I was right to continue to just give the "correct" 550 error response and not attempt to accept mail to unknown users to the domain in question in any way. The serverfault page you refer to is slightly helpful, but is only applies to individual users@domain, not the whole domain, and in any case I don't /want/ to reject all mail to that domain because I have standard users (postmaster, webmaster, admin, abuse, security etc) for whom I want to accept mail.
So, I will contnue to reject, but I'm still searching for a way to customise the rejection message. If anyone has any ideas, I'd welcome them.
I missed that the serverfault article was for specific users.
I had another google; this seems more promising, perhaps?
https://www.howtoforge.com/configure-custom-postfix-bounce-messages links to... http://www.postfix.org/bounce.5.html
good luck Steve