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

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

10.1 IN PTR moxmail1.mox1.example.
10.2 IN PTR moxmail2.mox2.example.
10.3 IN PTR moxmail3.mox3.example.
20.1 IN PTR postfixmail.postfix.example.
30.1 IN PTR dns.example.