Hi,
2822 suggests (section 3.6.4, page 25) using IP address, system time and
something like a unix pid for message-ids. I'm not entirely happy with
that advice: It says the msg-id MUST be unique, but goes on to suggest
an algorithm that does not generate unique results any more.
The observation that "no two hosts use the same domain name or IP
address at the same time" is regrettably not true. Addresses like
192.168.100.100 are in use by countless machines, and that two of them
might send mail at the same time, from the same "pid", isn't
far-fetched. Granted, it's not something that'll happen every five
minutes, but the net is big and keeps growing.
The same applies to localhost.localdomain and OS-default hostnames like
good old darkstar.frop.org.
Ideally, a revised document should suggest using a secure hash of the
message text on the LHS and the ip or domain on the RHS. At least it
should warn of the problems caused by 192.168/16 and its friends.
(I hope this message doesn't reopen an old DRUMS wound.)
--Arnt