I think you're making a couple of unjustified assumptions here:
First, you shouldn't assume that all the other hosts on a local subnet
are under the same administrative control as the host with the MTA in
question (think of a host in a colocation facility), and therefore you
also shouldn't assume that you should allow mail relaying to or from
other hosts on the same subnet by default.
Second, you can't assume that an administrator can or will put all hosts
he wants to allow relaying to or from on the same subnet as the MTA.
It's not only practical to require the sysadmin to enumerate which hosts
or networks he wants to permit relaying from, it's the only solution
that gives the sysadmin the required level of control over relaying
based on client IP addresses.
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