Guidelines for headers in original message


What are the general guidelines for headers and their values that are shown
in the original message of an email? I'm particularly interested in the
'Received from: foo by bar'. Do people generally append the actual
servernames along with their IP addresses for all the hops? Or, do they
make the mails come out from a common relay server.

Also, if it's fine to make mails come from a common relay, is it a bad idea
to set $myhostname to foo.relay for all the mails from foo domain, instead
of showing all the hops? I've seen people put this as $myorigin. Hence, the