At 08:53 17-11-2007, John C Klensin wrote:
I'm working this weekend on what I hope will be the last version of
2821bis prior to IETF Last Call, so, if you think such a mention
would be useful, please propose text and where it should go.
I have not come across any cases where the empty return path is
rewritten. If Ned thinks it might be a problem, some text could be added.
In Section 22.214.171.124:
"In some types of reporting messages for which a reply is likely to cause a
mail loop (for example, mail delivery and nondelivery notifications), the
reverse-path may be null (see Section 3.6)."
That provides some guidance on how to avoid a mail loop.
In Section 4.5.5, Messages with a null reverse-path:
"Implementors of automated email processors should be careful to make
sure that the various kinds of messages with null reverse-path are
handled correctly, in particular such systems SHOULD NOT reply to
messages with null reverse-path."
I read that as a warning about the null reverse-path, i.e. not to
rewrite the address or else the message may generate a reply.