The problem with specifying dynamic is that it isn't what we care
about.
Really, I don't care if an IP address is dynamic, however that's
defined. (Changes daily? weekly? There are DHCP-assigned addresses
that change less often than some supposedly "fixed" IP addresses.)
What I care about is whether or not the IP address owner thinks that
email should ever emanate from that IP address.
For instance, I'd want "This is a webserver that's not supposed to
emit email directly" on the list, while "This is a business-class
service customer who is welcome to emit email directly" not to be on
the list.
The general case, of course, is to list precisely which protocols the
IP address ought (not) to be initiating in general; one possible
reason for handling this via something suggestive (rather than just
blocking the port) is to allow sufficiently-clued servers to accept
connections (with authentication, perhaps) even though the general
case is otherwise.
Besides, when I'm feeling sufficiently paranoid, I'll do things like
run sshd on Port 25 with a banner that looks like a mailserver, just
in case somebody finds an exploit against the version of sshd I'm
running, how likely are they to try it against every mailserver they
see?
Seth
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