[exim] Receive mail on interface without address

My intention is to setup a backup server copying all mail sent to or
emanating from a mail server to individual message files on disk for
archiving. Note that I do not have any control over the main mail
server.

The archiving bit seems to work. The very minimal config is below.

However I would like to capture the mail off a mirrored switch port.

I have used 'ip link set eth1 up' to turn the interface on, and I can
see traffic using tcpdump.

However, as I understand it, Exim needs an address in local_address to
operate. I imagine I need to set local_address to the address of both
the mail server and the isp's server through which the local server
sends all its mail.

I tried setting eth1 to have the same ethernet address as the mail
server (the fact that it has a duplicate address seems to be hidden by
the port mirroring function on the switch thank heavens), but no mail
came through Exim.

Thoughts and advice much appreciated,
Rory

#######################################################################

MAILDIRPATH                 = /home/mail
SPOOLDIR                    = /var/spool/exim4
SYSFILTERON                 = 1
local_interfaces            = 192.168.1.3
exim_path                   = /usr/sbin/exim4

domainlist local_domains    = *
domainlist relay_to_domains = 
hostlist relay_from_hosts   = *

message_size_limit          = 1G
queue_only_load             = 50
deliver_queue_load_max      = 150
smtp_accept_max             = 50

queue_only
queue_run_max               = 30

host_lookup                 = no

rfc1413_hosts               = no

local_from_check            = no
local_sender_retain         = true
untrusted_set_sender        = no         # shouldn't need this

ignore_bounce_errors_after  = 7d
timeout_frozen_after        = 8d

freeze_tell                 = postmaster
spool_directory             = SPOOLDIR

.ifndef SYSFILTERON
    system_filter = /etc/exim4/sysfilter
    system_filter_user = 8
    system_filter_group = 8
    system_filter_pipe_transport = system_pipe_transport
.endif

acl_smtp_rcpt               = accept

begin acl
    # here in case we need it
    acl_check_rcpt:
      accept
          hosts = :

begin routers

route_all:
debug_print = "R: allhoster $local_part@$domain"
driver = accept
transport = transport_all

begin transports

transport_all:
  debug_print        = "T: maildir_home for $local_part@$domain"
  driver             = appendfile
  create_directory   = true
  directory_mode     = 0775
  directory_file     = $message_id # overrides default
  directory          = MAILDIRPATH/${substr_0_8:$tod_zulu}
  delivery_date_add
  envelope_to_add
  return_path_add
  mode               = 0666
  mode_fail_narrower = false

begin retry

*                      *           F,2h,15m; G,16h,1h,1.5; F,4d,6h
begin rewrite

begin authenticators

--
Rory Campbell-Lange
<rory@???>
<www.campbell-lange.net>