{"id":3517,"date":"2012-11-01T09:03:19","date_gmt":"2012-11-01T07:03:19","guid":{"rendered":"http:\/\/www.limilabs.com\/blog\/?p=3517"},"modified":"2019-03-15T17:10:14","modified_gmt":"2019-03-15T15:10:14","slug":"verp-variable-envelope-return-path-net","status":"publish","type":"post","link":"https:\/\/www.limilabs.com\/blog\/verp-variable-envelope-return-path-net","title":{"rendered":"VERP (Variable Envelope Return Path) in .NET"},"content":{"rendered":"

Variable Envelope Return Path (VERP) is a technique used to enable automatic detection and removal of undeliverable e-mail addresses<\/strong>. It works by using a different return path (also called “envelope sender” or MAIL FROM) for each recipient of a message.<\/p>\n

Bounce message processing problem<\/h2>\n

In general the hard part of bounce handling is matching up a bounce message with the undeliverable address<\/strong> that caused the bounce. <\/p>\n

Mail.dll email library<\/a> includes Bounce <\/em>class that analyses bounced emails<\/a> and extracts undeliverable Recipient address. The problem is that some bounced messages don’t contain this information or use invalid or unrecognizable format.<\/p>\n

If the mailing software could see that a bounce resulted from an attempt to send a message to bob@example.org<\/code>, 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 this email address, and how many bounces resulted, and if the proportion of bounced messages is too high, the address is removed from the list.<\/p>\n

There’s no need to process bounces with VERP<\/h2>\n

VERP solves bounce analyzing problem, by using a different return path for each email<\/strong>. <\/p>\n

Consider an email send from wikipedians@wiki.net<\/code> to bob@example.org. <\/p>\n

VERP changes the sender address to
\nwikipedians+bob=example.org@wiki.net<\/code> – it is still a valid email address, but it also contains information about the original recipient of the email. <\/p>\n

If the email is bounced, bounce message comes to a unique email address (wikipedians+bob=example.org@wiki.net<\/code>) that also contains information about the undeliverable recipient address<\/strong> (bob@example.org<\/code>).<\/p>\n

This way by creating catch all mailbox you can easily filter bounced email addresses.<\/p>\n

Return-path email header<\/h2>\n

Return-path email header contains email address that delivery failures (bounces) should go to. <\/p>\n

Return-path is added at the receiving end, from the SMTP’s “MAIL FROM” command. That’s why it is not the correct place for the VERP address, as in most cases it will be ignored and\/or overridden by real SMTP envelope sender (“MAIL FROM”). <\/p>\n

Proper VERP message<\/h2>\n

So how should the proper VERP message look like? Here are some points:<\/p>\n