Its certainly true that the issuer of a cert using such a regex represented
name matters greatly now. (I wondered about the same issue in another
thread) A third party CA is going to have a much harder time understanding
the local practices, and that represents a spoofing risk, though I suspect
it might be possible to communicate local practices to prevent that. A
workable scenario is that the certificate issuer is the email domain owner
who understands intimately the email local practices.
This is almost certainly a blind alley, and it'd help to back up and think
about what problem you're trying to solve.
It seems to me that even though the same S/MIME certificate is used for
signing and encryption, the two applications are quite different.
For signing, a random recipient having verified that the signature matches
the message body wants to check that an address in the certificate matches
the sender's address, typically the one on the From: line. In my
experience, even those of us with a zillion inbound addresses don't use
all that many addresses for outbound mail, so it's practical to enumerate
them all in certificates. Or if a sender does want to make an address on
the fly for which it does not yet have a certificate, if the CA is the
sender's domain owner, it should be straightforward invent to ask the CA
at that time for a certificate that matches the the address, and the CA
can use whatever local rules it wants to decide whether the desired
address is one that belongs to the requester and provide one or not.
This is extra work, but in the context of mail submission, it's not an
unreasonable amount of extra work.
For encryption of inbound mail, it doesn't matter what address is in the
certificate; if the recipient can decode the message it's the right one,
otherwise it's not. So the sender can ask the domain's key oracle for a
certficate for the address, the key oracle applies local rules and
provides a certificate that might have the exact requested address, or
might have another address that goes to the same recipient.
I'd think that something along these lines would not be particularly hard
to implement, and would require semantic changes neither to PKIX nor to
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