the meaning of header lines such as From and To in an enhanced header
are not defined since they will have been excluded from the normal
re-writing that takes place. But Mime related headers can be used.
Signing ANY headers is a Bad Idea (tm). All headers are considered
fair game by most MTA's. It's common to take any header and change the
line wrap for no reason.
Similarly, there's rampent case changes especially in strings like
"Message-ID" to "Message-Id", "CC:" to "Cc:". The list is endless.
If you want to sign headers, you'd best put them in the body of the
message.
braD