That is sort of the bone of contention.  The list adds 
  Sender

which points to the list.

The "Sender" header is "optional" unless the sender is not the same as "cfroM" (which -conceptually- really runs afoul the "Sender" in list headers, but that is unrelated).  Mail clients today seem to combine "From" and "Sender" to produce a "display 'From' " of something like "<listname> on behalf of <address>" where <listname> is pulled from "Sender:" and <address> comes from "From:"

Microsoft is ignoring the old RFC 822 meanings of the two headers (rfc 822 section 4.4).  However, they are not the first to do so.  Changing this will affect threading for replies.  It might work if *all* froms were rewritten, but that seems excessive.  And may be unworkable.

Below is MS on the issue ... 

https://blogs.msdn.microsoft.com/tzink/2016/11/02/troubleshooting-the-red-suspicious-safety-tip-for-fraud-detection-checks/



________________________________________
From: LISTSERV Site Administrators' Forum <[log in to unmask]> on behalf of [log in to unmask] <[log in to unmask]>
Sent: Tuesday, December 6, 2016 9:46 AM
To: [log in to unmask]
Subject: Modify "From:" mail header

Hi List,


When a user sends a message to a list, the message that gets distributed by ListServ still contains the mail address from the original sender in the "From:" mail header.


Is there a way to modify the "From:" header, so that it shows the list address, rather than the address of the original sender?


Kind regards,

Lucas

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