The mailing list for listmasters using Sympa

[Steve Shipway <address@concealed>]

> > I thought that the list-owner(s)' email address was for members to get
> > help from the list-owner(s). Has this line been added recently to
> > address dmarc problems or has this line always been part of email
> > headers? What does it mean or try to do?

The Sender: header is very important for list servers, and has been defined 
for decades.

This header is mandated by the SMTP RFCs as the 'Secretary scenario'.  
Basically, Sympa is like your secretary -- it sends out the emails on your 
behalf.  So, while the email is *From* you, it was *Sent* by Sympa. 

This is an important distinction when you are looking at things like SIDF 
(otherwise known as "evil bastard offspring of Microsoft and SPF") and DKIM, 
as well as S/MIME signatures.  If the Sympa server is in a different domain 
or different server from the original sender then it can trigger security 
warnings.

The reason is that, if you are validating the source email address against 
the SPF valid IP addresses for the domain, you need to compare against the 
Sender (if it exists), not the From.  Otherwise, you'll see a message from 
address@concealed actually coming from the mail servers of 
address@concealed and flag it as a forgery.

This is the root of the DMARC problems, because the DMARC RFC forgot about 
this and mandated DKIM checks against the From header regardless, even if the 
Sender header exists.  As a result, we have to break the SMTP RFC by changing 
the From header (rather than just adding a Sender header) if we want to pass 
DMARC checks.

Steve

----

Steve Shipway |  Senior Email Systems Administrator 
Phone: +64 9 302 0515 Fax: +64 9 302 0518 
Freephone: 0800 SMX SMX (769 769) 
SMX Limited: Level 15, 19 Victoria Street West, Auckland, New Zealand 
Web: http://smxemail.com
_____________________________________________________________________________

This email has been filtered by SMX. For more info visit http://smxemail.com
_____________________________________________________________________________

BEGIN:VCARD
VERSION:3.0
BDAY:1969-08-21
EMAIL;TYPE=work,pref:address@concealed
FN:address@concealed
ORG:SMX;Mail
PRODID:-//Open-Xchange//7.8.2-Rev13//EN
REV:20170111T082639Z
N:Shipway;Steve;;;
TEL;TYPE=cell,pref:021753189
TEL;TYPE=voice,home,pref:09 26 525 88
UID:b7b6b608-8ebe-4021-911a-189a532d4a2c
END:VCARD