Variable envelope return path

Variable envelope return path (VERP) is a technique used by some electronic mailing list software to enable automatic detection and removal of undeliverable e-mail addresses. It works by using a different return path (also called "envelope sender") for each recipient of a message.

Any long-lived mailing list is going to eventually contain addresses that can't be reached. Addresses that were once valid can become unusable because the person receiving the mail there has switched to a different provider. In another scenario, the address may still exist but be abandoned, with unread mail accumulating until there is not enough room left to accept any more.

When a message is sent to a mailing list, the mailing list software re-sends it to all of the addresses on the list. The presence of invalid addresses in the list results in bounce messages being sent to the owner of the list. If the mailing list is small, the owner can read the bounce messages and manually remove the invalid addresses from the list. With a larger mailing list, this is a tedious, unpleasant job, so it is desirable to automate the process.

Unfortunately, most bounce messages have historically been designed to be read by human users, not automatically handled by software. They all convey the same basic idea (the message from X to Y could not be delivered because of reason Z) but with so many variations that it would be nearly impossible to write a program to reliably interpret the meaning of every bounce message. RFC 1894 (obsoleted by RFC 3464) defines a standard format to fix this problem, but support for the standard is far from universal. However, there are several common formats (e.g., RFC 3464, qmail's qsbmf, and Microsoft's DSN format for Exchange) that cover large proportion of bounces.

Microsoft Exchange can sometimes bounce a message without providing any indication of the address to which the original message was sent. When Exchange knows the intended recipient, but is not willing to accept email for them, it omits their address. If a message is sent to joe@example.com and the server knows that this is Joe User, it will bounce the message saying that the message to Joe User could not be delivered, leaving out the joe@example.com address altogether. VERP is the only viable way to handle such bounces correctly.

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