Explaining VERP

Question

I am in the middle of making a bounce detection program, that reads bounce mails. Our setup is that we are sending out emails, adding a noreply@domain.tl to the sent mails. Some of the recipients does not exist anymore and therefor we want to read the bounce, and detect whom it was sent to. I have crumbled over this for a day or two now doing my googling and so. Now I have stumbled over something named VERP, Variable envelope return path. But is there someone here who can explain the technic to me in "human" terms?

I understand it in this way: send a mail to a recipient with a "FROM:" header set to noreply+recipient+recipient_domain.tl@domain.tl.

But how do the recipient mail server deduct the return address in the FROM header if the recipient does not exists? Does it deduct it from the Return-Path instead set in the sent mail?

Hope someone understands my question, and can explain me how it works, or maybe tell me an alternat way of doing it.

Answer 1

Imagine this:

You send a newsletter to :

• Bob@Example.org
• Jane@Example.org
• Steve@Example.org

Bob and Jane addresses are no longer existent.

Therefore for each mail address that you target you generate a unique Return-Path: address like so :

• For Bob : 1@bounce.yourdomain.com
• For Jane : 2@bounce.yourdomain.com
• For Steve : 3@bounce.yourdomain.com

Then checking your catch-all account on bounce.yourdomain.com (programatically) you find two emails :

One for 1@bounce.yourdomain.com and one for 2@bounce.yourdomain.com.

You previously stored somewhere that 1 belongs to bob and 2 to Jane so you remove them from the newsletter list.