[exim] Prevent email routing by default

Hi.

Using rbl_domains has significantly reduced the number of spams routed
through my server. However, there still remain several 10,000 emails
pouring in daily.
Is there some configuration option that I can use to prevent any
emails being routed via the server to non-local (i.e. remote ) email
addresses, unless the email originates from a FQDN that I explicitly
authorize using host_accept_relay?
Incidentally, in the line
host_accept_relay = 127.0.0.1 : ::::1
what does the trailing "1" stand for? What am I allowing by including
this value that is included in the default exim.conf file?

TIA,
Joseph



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