It seems likely that if this header should become popular, malware
would be changed to take advantage of that, and to use compromised
machines to spoof sender-auth headers within their own domains... so
this is a real threat that needs to be addressed. And it seems to me
that (1) is the right way to do it. So there should be something in
security considerations describing this problem, and suggesting (1)
a way to deal with it.
That's more than a "security consideration".
Have the same concerns been raised in other WGs for the possibility that
somebody might hack into an IMAP server and modify messages there?
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