Just to play devil's advodate:
Setting aside your view of poor MUA design for the moment, a
counter-argument to your point about reputation being evaluated at time
of reading rather than time of mailbox delivery is something that can
work to to the advantage of systems like DKIM which have the ability to
revoke a key:
- attacker manages to get some malicious content of some kind signed
under "example.com"
- message goes out
- message arrives at victim's inbox, but is as yet unread
- example.com discovers its vulnerability, figures out which key was
used, revokes the key
- MUA attempts to validate DKIM but finds that the key used to sign was
revoked and refuses to display the content
That's one place where checking authentication at the MUA works. There
are arguments in the appendix to which I referred you, though, which
possibly outweigh the benefits of this scenario though.
And the counter-argument to this is that someone from a negative
reputation sends something out which lands at the inbox, but by the time
the MUA gets around to opening it, the reputation has swung neutral or
even positive, so the reputation system has provided no benefit.
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