Content-type: text/plain; charset=us-ascii (Plain text)
Content-type: text/plain; charset="us-ascii"
are completely equivalent
DKIM has never tried to understand the semantics or syntax of each
header. First off, doing this properly is very hard as there are a
large number of "equivalent" header variations, but second off, it
would make DKIM brittle against future headers that might have the
same characteristics you describe.
Mark.
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