handling is matching up a bounce message with the undeliverable address that caused the bounce. If the mailing list software can see that a bounce resulted from an attempt to send a message to , then it doesn't need to understand the rest of the information in the bounce. It can simply count how many messages were recently sent to , and how many bounces resulted, and if the proportion of bounced messages is too high, the address is removed from the list. While bounce message formats in general vary wildly, there is one aspect of a bounce message that is highly predictable: the address to which it will be sent. VERP takes full advantage of this. In a mailing list that uses VERP, a different sender address is used for each recipient. The mailing list manager knows that it sent a message from X to Y, so if a bounce message is received at address X, it can only be because address Y was undeliverable, because nothing was sent from X to any other address. Thus the important information has been extracted from the bounce message, without any need to understand its contents, which means the person in charge of the list does not need to deal with it manually. Origin The first serious advocate of this solution, and the originator of the term VERP to describe it, was Daniel J. Bernstein, who first put the idea into practice in his qmail MTA and ezmlm mailing list manager. Example Assume there is a mailing list called wikipedians@example.net and that an individual, bob@example.org has subscribed to it, but later on, Bob has left example.org, so his address is no longer valid. Consider what happens when someone sends a message to the list. Without VERP Without VERP, the mailing list manager might send a message with the following characteristics: envelope sender: wikipedians-owner@example.net recipient: bob@example.org This would result in a bounce, generated by the MTA of either example.net or example.org, with the following characteristics: envelope sender: empty recipient: wikipedians-owner@example.net contents: example.org was unable to deliver the following message to bob: ... The mailing list manager can't be expected to understand the contents of this bounce, and can't deduce anything from the recipient address because hundreds of other people besides Bob were also sent messages from wikipedians-owner@example.net. With VERP With VERP, the original message would be different: envelope sender: wikipedians-owner+bob=example.org@example.net recipient: bob@example.org The