[Exim] local network lookups.

I have a question regarding DNS lookups. in my log I see this:

<cut>
H=(registrar3) [255.255.255.255]
</cut>

This machine is part of the local domain, and Exim is on the DNS server, and
this machine is in the proper DNS records.

I have in my configure:

host_lookup = *
host_accept_relay = "x.175.7.0/24 : \
                      x.6.130.0/24 : \
                     x.6.131.0/24 : \
                      x.6.129.0/24 : \
                      x.6.128.0/24 : \
                      255.255.255.255/32"

These are the 5 class C's we control.

Why do I get some local machines, or anyone for that matter, showing up as
255.255.255.255 ?
Should I be rejecting mail from 255.255.255.255?