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.
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.
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.
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.
Without VERP, the mailing list manager might send a message with the following characteristics: This would result in a bounce, generated by the MTA of either example.net or example.org, with the following characteristics: 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.
A mailing list using VERP, on the other hand, must send the entire message body repeatedly, which leads to an overall increase in bandwidth usage.
This way you can gain much of VERP's tight bounce control and accurate feedback without incurring the processing and network overhead every time.
Another problem with VERP (and with any automatic bounce handling scheme) is that there are MTAs on the Internet that fail to follow basic SMTP standards.
Systems that implement greylisting work fine with VERP if the envelope sender follows the above-mentioned format.