All of this incurs non-trivial per message overhead. Issuing a DNS query,
and/or querying the local account database, is not free, and the penalty
now applies to every message. A judgement call needs to be made whether
the additional overhead is justified by the value added.
I'm aware of all of this. However all (except sender address verification)
the overhead will still happen, but later in delivery chain, in which case
A second time. Twice as much overhead, now.
As the old saying goes: build an idiot-proof process, and nature will
evolve a better idiot.
Do you mean we shouldn't build idiot-proof processes because of this?
A completely idiot-proof solution is often impossible. Nearly all the time
this is reduced to a judgement call, whether the extra bang for the buck is
worth the additional penalty.