DELIVTERMS: Return-path/Envelope sender

First in a series: DELIVTERMS is where I'll explain words, terms and concepts relating to deliverability technology and best practices. Useful, or no? Let me know in email or in the comments! And thanks.

In today's deliverability terminology blog post, allow me to explain the Return-path address.

This is also called the envelope sender address/domain, bounce address or domain, or the 5321 FROM or MFROM.

What is it: Not to oversimplify it too much (hopefully), but email message have two from addresses. There’s what you normally think of as the from address – this is specified in the “from:” header in your email message and it shows up in your email client, webmail client or mobile email application. There’s also a second from address, called the return-path or envelope sender address. After selecting “view all headers” or raw email source, search for “return-path” to find this header and its contents.

Mail servers transmit this “envelope sender” address, which is meant to indicate who the original sender of the message is, to each other when sending your email message along to the recipient. It’s called “envelope sender” because it’s not transmitted as part of the body of the email (the body being “inside the envelope”) but rather as part of the SMTP transmission itself. Like perhaps how a postal service machine scans the outside of a message (the envelope) to tell where it’s coming from and where it’s going, without reviewing the contents of the email message.

This address is where bounce (non-delivery) notifications relating to your email message are supposed to be sent to.

Individual one-to-one emails will have your personal email address in both the return-path and from address. For email marketing and CRM systems, they will often customize this address to better capture notifications of undelivered emails (aka bounces), which is why I often refer to this address as the bounce address (and/or its domain name as the “bounce domain”). A lot of these email systems use a mechanism called “Variable Envelope Return Path” (VERP) to encode the return-path address in a way to track bounces more reliably. “Variable envelope” means that the bounce address (return-path) changes (varies) for each recipient. A unique code for each recipient makes it easier to log and categorize which message, to which recipient, generated a particular bounce. Thus, “variable envelope return path” is another way of saying, “a per-recipient varying bounce address.”

Calling this the 5321 FROM, MAIL FROM or MFROM comes from RFC 5321, one of the internet specification documents (called RFCs) governing how SMTP email works. Various RFCs have specified, clarified, improved and/or changed SMTP specifications over the years. For example, going all the way back to RFC 821, this bounce tracking address was referred to as the “reverse path.”

The company once called "Return Path" is named after this header. (Return Path was purchased by a company called Validity in 2019.) Return Path's original product was an "email change of address" service.

I am sure somebody will find something to nitpick in my overly simplified, but still perhaps a bit too brief definition that undoubtably doesn't explain everything about this term. Have at it in comments, nerds! And I hope the non-experts out there found this useful.

Want to learn more about deliverability terminology? If so, be sure to visit the DELIVTERMS section here on Spam Resource.

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