PKI Sign and Encrypt the message body/list of addresses. Since the list
is going to end up in an automated process, the need to do the
decryption and signature verification is not that big an issue to build
in. The Signature validates the list while the encryption protects the
addresses themselves from interception of the message.
dkim.org.
or, perhaps, goodmailsystems.com -- as far as I know, they are the only vendor
with a solution that includes rfc2821.RCPT-TO in the hashed data for the
signature.
(Disclosure: they are a client.)
d/
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net