At 16.23 +0000 01-10-04, D. J. Bernstein wrote:
One obvious counterexample is the very widely used Delivered-To field,
which provides much more effective loop control than any IETF field.
I do not think IETF has any standard for loop control in e-mail,
except the priority value in MX records.
Common methods for loop control:
(1) Trace list on the message. We have this, in the
"Received:" header, but no spec how it should be used for
loop control. This is not as easy as it can seem, since a
message can legitimately come and go several times to the
same MTA, for example after mailing list expansion.
(2) Store, in the MTA, the Message-IDs of messages passing.
Can also be combined with checksums of message content.
This is the main method of loop control in Usenet News, but
is not used very much outside Usenet News. In Usenet News,
it has a bug - the same article is *not* accepted twice,
even if it is received a second time to a different
newsgroup. This can only handle for messages entering News
from e-mail.
Listserv has a very good capability of recognizing
duplicates of the same message, even if the Message-ID has
become corrupted by non-standard software like First Class
(First Class puts a new Message-ID on a message which comes
into First Class and is then sent out again). I am not sure
exactly how this is done.
--
Jacob Palme <jpalme(_at_)dsv(_dot_)su(_dot_)se> (Stockholm University and KTH)
for more info see URL: http://www.dsv.su.se/jpalme/