system_aliases:
driver = redirect
allow_fail
allow_defer
data = ${lookup{$local_part}nis{mail.aliases}}
domains = harkness.co.uk
# The domains line is required otherwise the address lookup is munged
# data = ${lookup{$local_part}lsearch{/etc/aliases}}
# user = exim
file_transport = address_file
pipe_transport = address_pipe

Note that there is only one line required to enable aliases for NIS.

# Note that format of the file /etc/mail/exim-pass is just username space and plaintext password:

joe letmesendmail

plain:
driver = plaintext
public_name = PLAIN
server_condition = ${if eq{$3}{${lookup{$2}lsearch{/etc/mail/exim-pass}{$value}fail}}{yes}{no}}
server_set_id = $2

login:
driver = plaintext
public_name = LOGIN
server_prompts = Username:: : Password::
server_condition = ${if eq{$2}{${lookup{$1}lsearch{/etc/mail/exim-pass}{$value}fail}}{yes}{no}}
server_set_id = $1

#cram:
# driver = cram_md5
# public_name = CRAM-MD5
# server_secret = ${if eq{$2}{${lookup{$1}lsearch{/etc/mail/exim-pass}{$value}fail}}{yes}{no}}
# server_set_id = $1