I dispute his conclusion that a failed signature means that the message will
be thrown in the trash. Most filters (and certainly any compliant with the
criteria being discussed) would quarantine mail with a failed S/MIME
signature rather than discard.
That's a bit like saying that "most ISPs" would quarantine mail from dubious
sources rather than simply rejecting it or discarding it. I agree with you
that it's more robust to quarantine suspect mail than to discard it, but
the latter practice is already widespread.
The second point I would make is that the mailing list software is not
immutable. In fact mailing list software is likely to be rapidly upgraded to
support anti-spam filters very quickly since the manual anti-spam moderation
is a significant burden on the list admin.
What you cite as "fact" may not be supportable. When a dozen or more
intermediaries might be involved in the handling of a message from sender to
recipient, and when many of these intermediaries leave little clue about what
modifications they might have made to a message (firewalls and virus filters
typicaly don't even leave a received field), it's difficult to track down the
culprits, even more difficult to get the vendors of those products to fix
them, and still more difficult to get the fixes deployed.
The problem of broken, obsolete MUAs that do not support S/MIME cannot be
allowed to represent a veto.
Perhaps not, but it does seem useful to realize that such MUAs exist.
I have no time at all for
so-called technologists who refuse to eat their own dog food.
Welcome to the real world. A huge number of so-called technologists are
forced to use Windows and Exchange even though they know better.