Here is an early link to a draft submitted to the IETF utilizing the DNS
SRV record to publish authorized clients for any protocol including
SMTP. This draft also suggests the label for this SRV record can be
used to access a TXT record for error reporting information. This was
to provide a foundation for a solution independent of how the
information is used. This takes advantage of existing DNS listing and
reporting features while not creating a record possibly confused as
pointing to an SMTP server.
http://www.mail-abuse.com/public/draft-dougotis-SRV-CAA-00.txt
Our name server was modified recently so it may take some time to access
to this link. If so, try-
http://204.152.185.196/public/draft-dougotis-SRV-CAA-00.txt
-Doug