On Wed, Apr 16, 2025 at 12:22 AM Wei Chuang <weihaw=
[email protected]> wrote:

> I think the question of whether to use the exclude-list or include-list
> approach comes down to X- headers handling.  There's a large proliferation
> of these headers particularly for security gateway forwarding flows.  I
> suspect the include-list approach will likely ignore X- headers, while
> exclude-list will by default include signing all of them.  The latter will
> be more sensitive to the rumored MTAs that delete arbitrary X- headers that
> will need help from the header algebra.
>

We need to be cautious here.  I think BCP 178 constrains us against doing
anything that might increase this proliferation, such as by making "X-"
header fields into a convenient way through DKIM2 signing.

-MSK
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