TLS Indicators in Internet Message Headers

Greetings,

A quote from RFC 3207, section 4.1.

 "A publicly-referenced SMTP server would probably
  want to accept any verifiable certificate from an
  SMTP client, and would possibly want to put
  distinguishing information about the certificate
  in the Received header of messages that were
  relayed or submitted from the client."

I was curious whether anyone on this list is familiar
with what existing implementations are doing
with respect to adding security indicators in
any of the messages headers when TLS is used?
I'd be interested in seeing example message
headers from real implementations, or documents
that describe the syntax being used.  

Has any standard has been proposed?

Thanks,
Daryl Odnert
Tumbleweed Communications
email: [email protected]