[Exim] SMTP AUTH usage?

Is it common to add a new header field for mails with an authenticated
sender? The latest exim testing release appears to support AUTH and
if I could make sure that mails with an authenticated sender contain
a certain header field and the rest doesn't, then I could hack my POP
daemon to support XSENDER and XAUTHLIST. Additionally, people without
a client that supports those extensions would still get that information.

Michael