How VERP Solves The Bounce Handling Problem
The hard part of bounce 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 user@example.com 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 user@example.com, 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 destination. 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.
Read more about this topic: Variable Envelope Return Path
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