I was having this discussion with someone off-list but...
Where I live, I am serviced by only one ISP. I get a discount by having
my services (business, home, cell, internet, etc) bundled by this one
provider and they sign all my messages. Choosing another provider etc.
may not be financially agreeable. I also know that there are spammers or
bots on this provider that take enjoyment out of using my name. So I
want to say- Trust my signature but expressly distrust my providers
signature if not also signed by me. Both messages, mine and the spammers
are genuine and unchanged, signed by my provider, but only my signed
messages are valid.
I see this as a feature.
I don't understand what the provider's signature is bringing to the
party in this case. Why is:
your signature plus a please-distrust-this-provider signature
your signature alone?
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