1. it is not so clear; on how to implement a mailing list exploder; which
is neither (cmiiw) "3.3. Relays on the receiving end" nor
"3.4. Relays on the sending end".
Current mailing list software (at least as far as I'm aware) resends
the message from the user and domain the software runs as/in;
no, that's not the case. actually domains are not topological at all;
so there's no such thing as the "domain the software runs as/in".
Perhaps my wording wasn't clear; what I meant to say was, the envelope
sender (MAIL FROM: parameter) of the exploded message will be set by the
mailing list software to an address under its own control. For example, I
would expect messages received from a list such as
have a Return-Path of "<something>@imc.org" (or possibly
"<something>@somehost.imc.org", etc.). Aside from simple mail aliases,
most mailing list software I'm aware of does this, if nothing else to make
bounce checking easier. And in cases where this isn't true, frankly I
think it ought to be.
oh, okay. yes, lists are expected to set mail from/return-path to an address
that can handle bounces - though strictly speaking it doesn't have to indicate
anything about where the list traffic "comes from".