This is one way to do it.
Another way is to have the FTC (or whichever *trusted* body administrates this) either cleanse lists on behalf of spammers, or give spammers MD5 hashes of the addresses.
The hashing approach is the most interesting: to test an address, you MD5 hash it and look for it on the list of hashes. Reversing this process is mathematically very difficult, so given a list of hashes a spammer cannot derive a list of addresses.
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