bounce, then, will be more useful: envelope sender: empty recipient: wikipedians-owner+bob=example.org@example.net contents: example.org was unable to deliver the following message to bob: ... From this bounce message the mailing list manager can deduce that a message to bob@example.org must have failed. This example shows the simplest possible method of matching a VERP to a list subscriber: the entire recipient address is included within the return path, with the at sign replaced by an equals sign because a return path with two at signs would be invalid. Other encoding schemes are possible. Software that supports VERP CiviCRM EspoCRM Courier Mail Server Discourse exim, using a specialized Router/Transport combination ezmlm GNU Mailman G Suite Inxmail Mercury Mail Transport System mlmmj Mahara Mailchimp Maileon MediaWiki though mw:Extension:BounceHandler Moodle Plesk postfix qmail Sendmail, with a ruleset STEdb StrongMail Sympa Thexyz Zimbra Target Box NotifyBC AWS SES (Simple Email Service) Disadvantages The use of VERP requires each message to be sent once for every recipient, instead of once to each receiving SMTP server. This is because of a limitation of SMTP, which allows multiple recipient addresses to be specified in a single transaction, but only one sender address. When there are many subscribers in the same domain, a mailing list that is not using VERP can combine multiple deliveries into a single transaction. It connects to the appropriate server for the domain, gives the single sender address, the recipient addresses, and then sends the message contents only once. 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 inefficiency is usually not considered a big problem, especially by qmail users, since qmail always sends messages once per recipient, even when VERP is not being used. Some packages mitigate the impact of VERP by applying it selectively, for example a mailing list manager might only use VERP on 1 in 10 mailings. 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. VERP depends on the recipients' MTAs following the rule that bounces are sent to the envelope sender. This has been a standard requirement since the dawn of SMTP in