failure details is much more commonly implemented with variable envelope return path (VERP), while explicit requests for them are rarely implemented. NDRs are a basic SMTP function. As soon as an MTA has accepted a mail for forwarding or delivery it cannot silently delete ("drop") it; it has to create and send a bounce message to the originator if forwarding or delivery failed. Bouncing vs. rejecting Excluding MDAs, all MTAs forward mails to another MTA. This next MTA is free to reject the mail with an SMTP error message like "user unknown", "over quota", etc. At this point the sending MTA has to bounce the message, i.e. inform its originator. A bounce may arise also without a rejecting MTA, or as RFC 5321 puts it: "If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other reason, then it MUST construct an "undeliverable mail" notification message and send it to the originator of the undeliverable mail (as indicated by the reverse-path)." This rule is essential for SMTP: as the name says, it's a 'simple' protocol, it cannot reliably work if mail silently vanishes in black holes, so bounces are required to spot and fix problems. Silently dropping messages Today, however, it can be common to receive mostly spam emails, which usually uses forged Return-Paths. It is then often impossible for the MTA to inform the originator, and sending a bounce to the forged Return-Path would hit an innocent third party. In addition, there are specific reasons why it is preferable to silently drop a message rather than reject it (let alone bounce it): Heuristically filtered spam. Spam filters are not perfect. Rejecting spam based on content filtering implies giving to spammers a test environment where they can try several alternatives until they find content that passes the filter. Viruses and worms. Most times these are sent automatically from an infected machine. Since a bounce may contain a copy of the worm itself, it may contribute to its diffusion. Quoting again RFC 5321, section 6.2: "As discussed in Section 7.8 and Section 7.9 below, dropping mail without notification of the sender is permitted in practice. However, it is extremely dangerous and violates a long tradition and community expectations that mail is either delivered or returned. If silent message-dropping is misused, it could