WCM Forum - eMail: Reply-To vs. Return-Path

Zitat:

When the delivery SMTP server makes the "final delivery" of a
message, it inserts a return-path line at the beginning of the mail
data. This use of return-path is required; mail systems MUST support
it. The return-path line preserves the information in the <reverse-
path> from the MAIL command. Here, final delivery means the message
has left the SMTP environment. Normally, this would mean it had been
delivered to the destination user or an associated mail drop, but in
some cases it may be further processed and transmitted by another
mail system.

It is possible for the mailbox in the return path to be different
from the actual sender's mailbox, for example, if error responses are
to be delivered to a special error handling mailbox rather than to
the message sender. When mailing lists are involved, this
arrangement is common and useful as a means of directing errors to
the list maintainer rather than the message originator.

The text above implies that the final mail data will begin with a
return path line, followed by one or more time stamp lines. These
lines will be followed by the mail data headers and body [32].

It is sometimes difficult for an SMTP server to determine whether or
not it is making final delivery since forwarding or other operations
may occur after the message is accepted for delivery. Consequently,

any further (forwarding, gateway, or relay) systems MAY remove the
return path and rebuild the MAIL command as needed to ensure that
exactly one such line appears in a delivered message.

A message-originating SMTP system SHOULD NOT send a message that
already contains a Return-path header. SMTP servers performing a
relay function MUST NOT inspect the message data, and especially not
to the extent needed to determine if Return-path headers are present.
SMTP servers making final delivery MAY remove Return-path headers
before adding their own.

The primary purpose of the Return-path is to designate the address to
which messages indicating non-delivery or other mail system failures
are to be sent. For this to be unambiguous, exactly one return path
SHOULD be present when the message is delivered. Systems using RFC
822 syntax with non-SMTP transports SHOULD designate an unambiguous
address, associated with the transport envelope, to which error
reports (e.g., non-delivery messages) should be sent.

http://www.faqs.org/rfcs/rfc2821.html

Zitat:

When the "Reply-To:" field is present, it
indicates the mailbox(es) to which the author of the message suggests
that replies be sent. In the absence of the "Reply-To:" field,
replies SHOULD by default be sent to the mailbox(es) specified in the
"From:" field unless otherwise specified by the person composing the
reply.

http://www.faqs.org/rfcs/rfc2822.html