but you included as a local configuration problem
the case where the MUA needs to supply a return address.
MUAs typically pass a partially formed message to a submission agent
supplied with the MTA.
The submission agent figures out From, Return-Path, etc., from a
combination of sysadmin configuration and user configuration. See, e.g.,
What's the mechanism for ensuring that they agree on what such a return
address should look like?
They don't have to.
I use VERPs for my outgoing messages---e.g.,
---so that a program can reliably associate bounce messages with
original messages. The VERPs are created by qmail-inject, not the MUA.
The delivery of djb-dsn-* is handled by qmail-local, not the MUA.
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