Of course an extension of that idea would be to identify all the other ranges that are marked as unassigned or dubious, like 224. or 14. or 240. and so on. INTERNET PROTOCOL V4 ADDRESS SPACE and RFC3330 - Special-Use IPv4 Addresses might be a start here.
That's fairly complex, and prone to require a lot of
updating as ranges are reassigned. It would be simpler
to rely on an extant mechanism, such as DNS. For
example, require the IP address of the sending mail
server to be reverse-lookupable (PTR record in
in-addr.arpa).
Of course, then you'll be blocking mail from pretty
much 100% of Asia... however, it would be possible
to combine this with other techniques -- for example,
if the IP of the sending mailserver _isn't_ reversable,
check it against a whitelist, and if it's not there
greylist and tempfail it for N minutes, or apply
heuristics, or whatever.
Of course, all of that has pretty much nothing to do
with the email address in the From field, which can
generally be considered worthless for such purposes.
If you're checking an email address that someone
used to sign up for some service, just require
them to respond to a confirmation message. That
positively guarantees the address is valid.
$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
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