Re: DMARC and yahoo

Dave Crocker <> Mon, 21 April 2014 19:15 UTC

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Date: Mon, 21 Apr 2014 12:13:08 -0700
From: Dave Crocker <>
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To: Doug Royer <>,
Subject: Re: DMARC and yahoo
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On 4/21/2014 11:04 AM, Doug Royer wrote:
> If yahoo sends out an email from list-name@yahoo, then that is where the
> email is from.

Unfortunately, email is more complicated than that.  There are multiple 
actors, taking multiple roles.  The most important one to represent 
properly in the message is the author, shown in the rfc5322.from header 

At the top level, an author is communicating with recipients, and often 
they need to be able to reply to to that author, without copying the 
other recipients.  Mess with the original From field and the ability to 
reply is typically affected.

A mailing list is a mediator to this exchange, such that, yes, it takes 
formal delivery and formally posts a new message.  However the 
'original' author and the 'final' recipients still treat this as an 
exchange between them.  The recipients are aware of the role of the 
mailing list, but they do not consider the mail to be 'from' the mailing 

The mailing list is an originator, in formal terms, which is typically 
represented by the rfc5322.sender field.

Changing the rfc5322.From field to be different than what the original 
author created alters communication among the the participants.

> So it would be correct to set the from/sender in the email to be

from/sender?  those are two different fields.

> Why try to make it fake anything?
> If you want to preserve any digital signature in the original message,
> then send the original email as a mime body part in the forwarded message.

Encapsulating the original message into a subordinate message has some 
appeal.  It also well might have some additional attack vectors.  In any 
event, it's an end-to-end change, probably affecting every entity 
handling the message.


Dave Crocker
Brandenburg InternetWorking