[Exim] Unknown users and return-path/Resent-from

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Author: John Horne
Date:  
To: Exim Users List
Subject: [Exim] Unknown users and return-path/Resent-from
Hello,

Well I'm still having a problem with some headers! The latest situation is
when an external user mails someone here, the message hits the central
mailhub which then sends it on to the users file store on Novell file
servers. These run the Mercury MTA. If the local (to us) user then forwards
the message to one or more people (which all go via the mailhub), and that
new recipient may or may not forward it again to one or more people, then if
one of the new recipients is an unknown user the mailhub sends the failure
message back to the very original sender - who obviously has no idea about
the failed user and never sent the message to them in the first place!
Sort of:

 external                local        local       local
   user   ->  mailhub -> user    ->   user   ->   user    -> valid recipients
     ^          |
     |          |
     ------------
    (unknown local user failure message)


We (the postmasters) then get the message sent back to us asking why we had
sent them a failure message about someone they don't know.

The transport/director we have are:

unknown_user:           (transport)
  driver = pipe
  command = "TABLES/messages/unknown-user.sh"
  ignore_status
  return_output
  timeout = 10m
  user = exim


unknownuser:             (director)
  no_expn
  no_verify
  transport = unknown_user
  driver = smartuser


This has worked well for a long time now, but I am wondering if the
smartuser director needs a 'new_address' setting and if so, would the
$reply_address or $return_path be better? My understanding is that the
transport should be using the message headers (which is the $reply_address)
to send the failure message back, but as said it goes to the external user
rather than back to the local user on the file server.

I have noticed that the message from the file server contains the external
user in the From: field, it has the correct Resent- fields (i.e. they are of
local users), but that it also contains a Return-path: field which has the
external users address. I am surprised (and confused) as to how this has
managed to last all the way through since upon the messages original arrival
at the mailhub the Return-path header should have been taken off. The
envelope from is obviously going to change to the mailhub and then the file
servers, so how the external address remained is baffling.

Despite all of that, I thought that the message should have gone back to the
Resent-from: header address, which is a local user.

Thoughts, suggestions?

John.

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John Horne, University of Plymouth, UK             Tel: +44 (0)1752 233914
E-mail: jhorne@???
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