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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