I should have said, yes there is an SPF record for thirdparty.org so that SPF 
passes based on the EHLO domain.


From: Franck Martin [mailto:[email protected]]
Sent: 30 April 2015 17:35
To: John Mears
Cc: Dave Crocker; [email protected]
Subject: Re: [dmarc-ietf] DMARC,SPF, null senders, and indirect mail flow


On Apr 30, 2015, at 6:58 AM, John Mears 
<[email protected]<mailto:[email protected]>> wrote:

The rfc5321.MailFrom is empty:
MAIL FROM:<>

The ehlo field presented to the recipient from the third party relay contains 
the domain of the third party:
EHLO thirdparty.org<http://thirdparty.org>

For SPF to pass you can put a record on the domain 
(thirdparty.org<http://thirdparty.org>) of the HELO, but this will not align 
with senderorg.com<http://senderorg.com>

see the definition of RFC7208.MAILFROM and 
https://tools.ietf.org/html/rfc7208#section-10.1.3



The from header contains the original sender's email address:
From: John Doe <[email protected]<mailto:[email protected]>>

What may work, is to get dedicated sending IPs at the third party relay and ask 
them to put in the HELO a domain aligned with 
senderorg.com<http://senderorg.com>, otherwise DKIM sign the bounces. Note that 
postfix/sendmail can DKIM sign the bounces it creates.
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