Exim Configuration – Change Sender Address When Sending Mails Out of Local Network

The content of the Received: header is defined in Exim by the configuration option received_header_text. The default setting, from which you can see how your example is constructed, is:

received_header_text = Received: \
  ${if def:sender_rcvhost {from $sender_rcvhost\n\t}\
  {${if def:sender_ident \
  {from ${quote_local_part:$sender_ident} }}\
  ${if def:sender_helo_name {(helo=$sender_helo_name)\n\t}}}}\
  by $primary_hostname \
  ${if def:received_protocol {with $received_protocol}} \
  ${if def:tls_cipher {($tls_cipher)\n\t}}\
  (Exim $version_number)\n\t\
  ${if def:sender_address \
  {(envelope-from <$sender_address>)\n\t}}\
  id $message_exim_id\
  ${if def:received_for {\n\tfor $received_for}}

As for changing or remove the header. Beware some best practice advice lies ahead..

• Are you sure that you want to remove this information? It's presence allows you to track abuse reports much more easily. The exposure of your internal IP addresses is actually of fairly limited risk.

• Technically you can remove this first received header by using headers_remove but it's certainly not RFC friendly and there is a chance of creating mail loops.

• If you must mask the information then you would be best to do so by modifying received_header_text. For maintainability and the principle of least surprise, even if the MTA isn't performing any other functions, you probably want to make your changes as specific as possible. This would involve putting some additional conditions in those if statements for facts that you know will always be true, such as whether the sender has authenticated themselves.

Some rules are dumb because they cause more harm than good! And people advocating and creating them know that. I would care more about postmaster ethics than the dumb rules. And I hate dumb rules and obviously the people setting those rules. If you are like me, aware of consequences (see above) and want to screw the rules then sure,

1) Establish sender and recipient verification to secure yourself OR

2) Fail or freeze all null postmaster bounces. Do it with an exim system filter

system_filter = /etc/exim/screwtherules.exim
vi /etc/exim/screwtherules.exim
if $sender_address is ""
then
if ${lookup{${extract{2}{@}{$recipients}}}lsearch{/etc/localdomains}{yes}{no}} is "no"
then
fail text "Delayed bounce message ignored"
seen finish
endif
endif

3) OR create an ACL at acl_smtp_data check time to forward, fail or freeze the null bounce emails being sent to or not to specific hosts. You can extract received header info as described in https://grepular.com/Exim_Trick_to_Extract_Received_Header_IP_Addresses