Q
On our mailing application we are sending emails with the following header:
FROM: marketing@customer.com
TO: subscriber1@domain1.example
Return-PATH: bouncemgmt@ourcompany.example
The problem that we are facing is that some email servers will bounce back a message immediately and use the from or reverse path (marketing@customer.example) 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.
C
A
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: <coolstuff@mymailinglist.example>
To: <you@example.com>
Subject: Super simple email
Reply-To: <coolstuff-threadId=123@mymailinglist.example>
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 coolstuff-you=yourcompany.com@mymailinglist.example. The SMTP session might look like:
{S}220 workstation1 Microsoft ESMTP MAIL Service
{C}HELO workstation1
{S}250 workstation1 Hello [127.0.0.1]
{C}MAIL FROM:<coolstuff-you=yourcompany.com@mymailinglist.example>
{S}250 2.1.0 me@mycompany.com....Sender OK
{C}RCPT TO:<you@example.com>
{S}250 2.1.5 you@example.com
{C}DATA
{S}354 Start mail input; end with <CRLF>.<CRLF>
{C}From: <coolstuff@mymailinglist.example>
To: <you@example.com>
Subject: Super simple email
Reply-To: <coolstuff-threadId=123@mymailinglist.example>
This is a very simple body.
.
{S}250 Queued mail for delivery
{C}QUIT
{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: coolstuff-you=yourcompany.com@mymailinglist.example
From: <coolstuff@mymailinglist.example>
To: <you@example.com>
Subject: Super simple email
Reply-To: <coolstuff-threadId=123@mymailinglist.example>
This is a very simple body.
Now, let’s describe the different “FROM”s.
- The return path (sometimes called the reverse path, envelope sender, or envelope from — all of these terms can be used interchangeably) is the value used in the SMTP session in the
MAIL FROM command. As you can see, this does not need to be the same value that is found in the message 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 already exists in the message, then that header is removed and replaced by the recipient’s mail server.
All bounces that occur during the SMTP session should go back to the Return-Path address. 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.
- The FROM address is the value 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.
- 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 recipient of the newly composed email. The Reply-To value should not be used by any server. It is meant for client-side (MUA) use only.
However, as you can tell, not all mail servers obey the RFC standards or recommendations.
A
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.
C
I would point out that the primary concept not captured in this analogy is that the Return-Path header is added by the receiving mail server and not by the sender. So it’s more like this: you can write whatever address you want inside of the envelope, but to deliver it you have to take it to the post office and show them your drivers license (or other ID) and they put that address on the envelope before sending it. In other words, the Return-Path header is as trustworthy as the checks performed by the receiving SMTP server, where the others can be easily spoofed
The Wiert Corner - irregular stream of stuff 
Leave a comment