message with an SMTP error code. This would leave Jack's mail server (at store.example) the obligation to create and deliver a bounce. Terminology Bounces are a special form of autoresponder. Autoresponses (automatic replies) are mails sent by a program—as opposed to a human user—in reply to a received mail and sent to the bounce address. Examples of other auto replies are vacation mails, challenges from challenge-response spam filtering, replies from list servers, and feedback reports. These other auto replies are discussed in RFC 3834: auto replies should be sent to the Return-Path stated in the received mail which has triggered the auto reply, and this response is typically sent with an empty Return-Path; otherwise auto responders could be trapped in sending auto replies back and forth. The Return-Path is visible in delivered mail as header field Return-Path inserted by the SMTP mail delivery agent (MDA) (which is usually combined with a mail transfer agent, or MTA). The MDA simply copies the reverse path in the SMTP MAIL FROM command into the Return-Path. The MDA also removes bogus Return-Path header fields inserted by other MTAs; this header field is generally guaranteed to reflect the last reverse path seen in the MAIL FROM command. Today these paths are normally reduced to ordinary email addresses, as the old SMTP 'source routing' was deprecated in 1989; for some historical background info see Sender Rewriting Scheme. One special form of a path still exists: the empty path MAIL FROM:<>, used for many auto replies and especially all bounces. In a strict sense, bounces sent with a non-empty Return-Path are incorrect. RFC 3834 offers some heuristics to identify incorrect bounces based on the local part (left hand side before the "@") of the address in a non-empty Return-Path, and it even defines a mail header field, Auto-Submitted, to identify auto replies. But the mail header is a part of the mail data (SMTP command DATA), and MTAs typically don't look into the mail. They deal with the envelope, that includes the MAIL FROM address (a.k.a. Return-Path, Envelope-FROM, or "reverse path") but not, e.g., the RFC 2822-From in the mail header field From. These details are important for schemes like BATV. The remaining bounces with an empty Return-Path are non-delivery reports (NDRs) or delivery status notifications (DSNs). DSNs can be explicitly solicited with an SMTP Service Extension, however it is not widely used. Explicit requests for delivery