email mail - What is the behavior difference between return-path, reply-to and from?

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I had to add a Return-Path header in emails send by a Redmine instance. I agree with greatwolf only the sender can determine a correct (non default) Return-Path. The case is the following : E-mails are send with the default email address : [email protected] But we want that the real user initiating the action receives the bounce emails, because he will be the one knowing how to fix wrong recipients emails (and not the application adminstrators that have other cats to whip :-) ). We use this and it works perfectly well with exim on the application server and zimbra as the final company mail server.

On our mailing application we are sending emails with the following header:

FROM: [email protected]
TO: [email protected]
Return-PATH: [email protected]

The problem that we are facing is that some email servers will bounce back a message immediately and use the from or reverse path ([email protected]) instead to our bounce mgmt server. We want to know if we modify in the header the reply-to to be the same as the return-path if we will be able to catch all bounces.

Any other ideas are welcome?

We are using the following documents as references: VERP RFC Bounce Messages

SMTP Log Parsing to get Bounces

EDIT 1: A few more bits of information to see if we can get this resolve.

We want to know at what point the email server relaying the message will choose to use the reply-to versus the return-path. We have notice that when the first smtp server relaying the message gets rejected it sends it to the reply-to, but when it happens after one hop it sends it to the return-path.

Let's start with a simple example. Let's say you have an email list, that is going to send out the following RFC2822 content.

From: <[email protected]>
To: <[email protected]>
Subject: Super simple email
Reply-To: <[email protected]>

This is a very simple body.

Now, let's say you are going to send it from a mailing list, that implements VERP (or some other bounce tracking mechanism that uses a different return-path). Lets say it will have a return-path of [email protected] The SMTP session might look like:

{S}220 workstation1 Microsoft ESMTP MAIL Service
{C}HELO workstation1
{S}250 workstation1 Hello []
{C}MAIL FROM:<[email protected]>
{S}250 2.1.0 [email protected] OK
{C}RCPT TO:<[email protected]>
{S}250 2.1.5 [email protected] 
{S}354 Start mail input; end with <CRLF>.<CRLF>
{C}From: <[email protected]>
To: <[email protected]>
Subject: Super simple email
Reply-To: <[email protected]>

This is a very simple body.

{S}250 Queued mail for delivery
{S}221 Service closing transmission channel

Where {C} and {S} represent Client and Server commands, respectively.

The recipient's mail would look like:

Return-Path: [email protected]
From: <[email protected]>
To: <[email protected]>
Subject: Super simple email
Reply-To: <[email protected]>

This is a very simple body.

Now, let's describe the different "FROM"s.

  1. The Return-Path (sometimes called the Reverse-Path or Envelope-FROM -- all of these terms can be used interchangeably) is the value used during the SMTP session. As you can see, this does not need to be the same value that is actually found in the mail headers. Only the recipient's mail server is supposed to add a Return-Path header to the top of the email. This records the actual Return-Path sender during the SMTP session. If a Return-Path header is already exists in the email, then that header is to be removed, and replaced by the recipient's mail server.

    All bounces that occur during the SMTP session should go back to the Return-Path value. Some servers may accept all email, and then queue it locally, until it has a free thread to deliver it to the recipient's mailbox. If the recipient doesn't exist, it should bounce it back to the recorded Return-Path value.

    Note, not all mail servers obey this rule. Some mail servers will bounce it back to the FROM address.

  2. The FROM address is the value actually found in the FROM header. This is supposed to be who the message is FROM. This is what you see as the "FROM" in most mail clients. If an email does not have a Reply-To header, then all human (mail client) replies should go back to the FROM address.

  3. The Reply-To header is added by the sender (or the sender's software). It is where all human replies should be addressed too. Basically, when the user clicks "reply", the Reply-To value should be the value used as the recpient of the newly composed email. The Reply-To value should not be used by any server. It is meant for client side use.

    However, as you can tell, not all mail servers obey the RFC standards or recommendations.

Hopefully this should help clear things up. However, if I missed anything, let me know, and I'll try to answer.

for those who got here because the title of the question:

I use Reply-To: address with webforms. when someone fills out the form, the webpage sends an automatic email to the page's owner. the From: is the automatic mail sender's address, so the owner knows it is from the webform. but the Reply-To: address is the one filled in in the form by the user, so the owner can just hit reply to contact them.

Another way to think about Return-Path vs Reply-To is to compare it to snail mail.

When you send an envelope in the mail, you specify a return address. If the recipient does not exist or refuses your mail, the postmaster returns the envelope back to the return address. For email, the return address is the Return-Path.

Inside of the envelope might be a letter and inside of the letter it may direct the recipient to "Send correspondence to example address". For email, the example address is the Reply-To.

In essence, a Postage Return Address is comparable to SMTP's Return-Path header and SMTP's Reply-To header is similar to the replying instructions contained in a letter.

Edit: The original tutorial I linked to has gone 404. Here's another that's ok:

In short, a MUA is a user client that uses SMTP to send an email to an MTA. The MTA is a server that is responsible for routing the MTA to its destination. If that destination is another server, the MTA hands the email to an MDA. The MDA is a client on the server that uses SMTP to forward the email to the other server, which is also an MTA.

So what do you need to learn? If you want to write an MUA or MDA, you need to learn how to open a socket to another computer, send SMTP commands, and receive SMTP responses. If you want to write an MTA, you need to learn how to listen for socket connections on a port, receive SMTP commands, and send SMTP responses.

If you like Java, try the code on this page as a starting point for a client.

email smtp rfc email-client bounce