[Exim] Backup MX ACL

Hi,

I have a simple enough question regarding an ACL I am trying to create
for exim-4.10. It is an ACL for any mail accepted for the @mx_secondary
domain list (ie domains the server is backup for), I would like to
restrict accepting the mail for just any domain that has an MX record
pointing at my server as this is open to abuse.

Now I could use the "domains = ..." ACL option to restrict to all the
destination domains to ones I control however this list is large and
changes often so what I would like to know is there a way to restict the
destination @mx_primary to my network? Basically somelike the "hosts =
..." ACL option but for destination ip addresses.

Thanks,
    Vince.