[exim] Limiting system_aliases recipients to only "local" ad…

Hi all,

I have a pretty standard exim conf set up on FreeBSD (Exim version 4.71), system aliases (or groups) are configured thus:

system_aliases:
driver = redirect
allow_fail
allow_defer
data = ${lookup{$local_part}lsearch{/etc/aliases}}
user = mailnull
group = mail
file_transport = address_file
pipe_transport = address_pipe

I would like to limit senders to these groups/aliases to only local addresses (i.e. sender1@???, othersender@???... but _NOT_ outsider@???).
How can I do this?

Thanks in Advance,
Janelle.