On Fri, 29 Jul 2011 09:48:44 EDT, William Herrin said:
Correction: It's a standard way to denote that "this mail is a bounce report."
Correction to your correction: What the RFC actually says: 4.5.5. Messages with a Null Reverse-Path There are several types of notification messages that are required by existing and proposed Standards to be sent with a null reverse-path, namely non-delivery notifications as discussed in Section 3.7, other kinds of Delivery Status Notifications (DSNs, RFC 3461 [32]), and Message Disposition Notifications (MDNs, RFC 3798 [37]). All of these kinds of messages are notifications about a previous message, and they are sent to the reverse-path of the previous mail message. (If the delivery of such a notification message fails, that usually indicates a problem with the mail system of the host to which the notification message is addressed. For this reason, at some hosts the MTA is set up to forward such failed notification messages to someone who is able to fix problems with the mail system, e.g., via the postmaster alias.) It's *not* just "bounce reports" (in particular, DSNs and MDNs are not non-delivery (bounce) messages in the sense of section 3.7, and both can be generated in response to *successful* deliveries). generated for *successful* deliveries).