When the receiving mailserver says (as is recorded in the sending mailservers log), that is has accepted the message for delivery or somthing similar (Example: "stat=Sent (gA2LC60L027839 Message accepted for delivery)"), then I'd guess they are under some obligation. They certainly cannot send it to/dev/null. You can decide to put a letter in the bin without reading it, the mailman cannot. I am sure some RFC does forbid giving a false statement as to what is done with accepted mail.
I think the only acceptable policy is refusing to accept messages, either with 'user unknown' or something else, e.g. telling the sender of the refused message to look at a webpage, where it says something like: this account is suspended for now. This is very easy to do, in any case in sendmail.
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When the receiving mailserver says (as is recorded in the sending mailservers log), that is has accepted the message for delivery or somthing similar (Example: "stat=Sent (gA2LC60L027839 Message accepted for delivery)"), then I'd guess they are under some obligation. They certainly cannot send it to /dev/null. You can decide to put a letter in the bin without reading it, the mailman cannot. I am sure some RFC does forbid giving a false statement as to what is done with accepted mail.
I think the only acceptable policy is refusing to accept messages, either with 'user unknown' or something else, e.g. telling the sender of the refused message to look at a webpage, where it says something like: this account is suspended for now. This is very easy to do, in any case in sendmail.