daemon@decwrl.UUCP (12/20/83)
From: John Covert <castor::covert> "Been here once before" is not appropriate for loop detection when sending mail to someone who travels around the net and may have forwarded his incoming mail to another site. A message should be allowed to pass through the same site a reasonable number of times, say four, before it is considered to be in a loop.
mark@cbosgd.UUCP (Mark Horton) (12/23/83)
On the other hand, if you store a recent history on your machine (like the mechanism netnews uses), you can store the pair (Message-ID, Destination) locally, and if it repeats, you know you have a loop. This assumes that (a) the message has a Message-ID (it isn't required by RFC822, although some programs, like sendmail, add one if the message doesn't already have one) and that somebody hasn't written an idiot reply command that duplicates the Message-ID (amazingly enough, this isn't forbidden by 822).
gnu@sun.uucp (John Gilmore) (12/27/83)
"Been here once" also doesn't work if mailing lists are used. For example, a message sent on MIT-MC.ARPA to UNIX-EMACS@CMU-CS-C.ARPA has to be able to return to MIT-MC.ARPA for recipients who were specified in the mailing list on CMU-CS-C.ARPA. This could happen several times, if there are nested communities of interest, or local delivery lists.