$ORIGIN 28.172.in-addr.arpa.
$TTL 5m

@ IN SOA dns.example. hostmaster.example. (1 0m 0m 0m 5m)

10.1 IN PTR moxacmepebble.mox1.example.
20.1 IN PTR moxmail2.mox2.example.
30.1 IN PTR dns.example.
40.1 IN PTR acmepebble.example.
50.1 IN PTR test.example.