Here is an example of an Exim configuration file that will:

Accept messages FOR the domain example.com.

Accept messages FROM itself (127.0.0.1) and two other ranges of IP addresses (10.0.0.0/8 and 192.168.0.0/16).

This config has very few features but is still functional enough to work (on a test system that is not accesible from the Internet).

This config also assumes that if you wish to send an email to This e-mail address is being protected from spam bots, you need JavaScript enabled to view it , that alice has a valid account and mailbox on the Exim server itself. 

# start of /etc/exim/exim.conf

######################################################################
#                       EXIM CONFIGURATION                           #
######################################################################
acl_smtp_rcpt   =       acl_check_rcpt

######################################################################
#                       ACL CONFIGURATION                            #
######################################################################
begin acl

acl_check_rcpt:
 

  accept  domains       = example.com : *.example.com         

  endpass

  verify        = recipient

  accept  hosts         = 127.0.0.1 : 10.0.0.0/8 : 192.168.0.0/16

  deny    message       = relay not permitted

######################################################################
#                      ROUTERS CONFIGURATION                         #
######################################################################
begin routers

dnslookup:
  driver = dnslookup
  domains = ! example.com : ! *.example.com 
  transport = remote_smtp
  ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
  no_more

localuser:
  driver = accept
  check_local_user
  transport = local_delivery
  cannot_route_message = Unknown user

######################################################################
#                      TRANSPORTS CONFIGURATION                      #
######################################################################
begin transports

remote_smtp:
  driver = smtp

local_delivery:
  driver = appendfile
  file = /var/mail/$local_part
  delivery_date_add
  envelope_to_add
  return_path_add
  group = mail
  mode = 0660

######################################################################
#                      RETRY CONFIGURATION                           #
######################################################################
begin retry

# Address or Domain    Error       Retries
# -----------------    -----       -------

*                      *           F,2h,15m; G,16h,1h,1.5; F,4d,6h
# end of /etc/exim/exim.conf