Public reply, because private are blocked.
uses rev-dns TXT RRs to let admins document which IP addresses are
supposed to act as MTA (as well as to document which addresses are
supposed not to send mail).
The difference is an ISP may "permit" any customer to act as an MTA. The
ISP does not want to decide about what to permit or deny. But the ISP
may be willing to provide more information about an address so other
people can decide if they want to behave differently depending on the
type of connection.
For example, an IRC operator may decide not to permit anyone using a
dynamic address to connect, or a Web site may want to send smaller pages
to users on low bandwidth connections.
In fact, you won't be able to reply to this email unless
_perm._stmp_srv.180 IN TXT "1"
into the DNS for you.
If an ISP permits every IP address to use any service, what does it
accomplish? If an ISP didn't want to permit its users to access SMTP,
the ISP would just block port 25 at the network layer.