[Pkg-exim4-users] Sender rewriting? MS Exchange showing 'On behalf of ...' instead of usual From name

[Ron Leach]

List, good afternoon,

We use Exim and Dovecot on Wheezy LTS.  Although we've used a standard configuration for Exim, I think we must have somehow either upset something or used a wrong setting in the debian conf dialog.  Further, the Exim system here is used as the MTA for internal mail among a few users, and also for outbound mail which is forwarded to a remote smarthost (we used the 'smarthost' setting in its configuration dialog).  That smarthost, in turn, is our own, and is another Exim instance running on a hosting service (Jessie, so Exim 4.84-2), that we rent.  That outbound smarthost employs authenticated submission, and all the messages from our office Wheezy LTS MTA are authenticated using a single login name, "admin", with its own secret (in passwd.client). 

One of our regular recipients is a multinational which uses MS Exchange and Outlook to manage its email.  Recently we are seeing, from their replies to our messages, that our messages are being displayed on their systems as being from

> -----Original Message-----
> From: admin at ourdomain.eu [mailto:admin at ourdomain.eu] On Behalf Of Ron Leach

>From headers in some test messages, we can see that
(i)   The From field remains as set by the MUA 
      - Ron Leach <ronleach at ourdomain.eu>
(ii)  The Sender field is set by Exim to
      - admin at ourdomain.eu
(iii) The Return-Path is set to
      - admin at ourdomain.eu
(iv)  Messages received by MS Exchange also show that exim sets
      - envelope-from <admin at ourdomain.eu>
      (though this is not apparent from the headers reported at other destinations)

Until this anomaly became apparent to me on some replies from that counterparty who uses MS Exchange, I had not noticed that our configuration was causing "sender" and "return-path" to be set to something other than the From name.

I don't know what might be causing those settings to differ from the simple 'From' set by the MUA.  Might it be something to do with the use of "admin" as the authenticator for the smarthost?

To be honest, I'm not at all sure what the interplay between From, Sender, Return-path, and Envelope-from is, ought to be, or is trying to achieve.  I looked at Exim ch31 about 'rewriting addresses' and though it explains *how* to set different addresses, I didn't understand much from that about *why* one should or shouldn't.  I've also read through 
 https://wiki.debian.org/PkgExim4UserFAQ
and, because it also talks about headers,
 https://wiki.debian.org/GmailAndExim4
but felt that things still were not clear enough for me to be confident of making any alterations to the configuration.  I thought I had better ask for comment or advice, before breaking things.

May I ask:

Why is Exim setting sender and return-path and envelope-from to differ from any names used by the MUA?

Does anybody else have any knowledge of why MS Exchange systems may be showing 'From' as 'On behalf of ...' in this situation?

I'd be extremely grateful for any comments - any at all,

regards, Ron