"ISPs are under no obligation to bounce email, ... (Score 1)

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.