> I've never heard of any provider making someone turn over their mailing lists. Do you have any further information about when this has occurred?

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I didn't mean to imply that was what happened. I said that they'd have to if it's to be checked.

My thoughts were that you're either going to have to trust the accused spammer - in which case you can turn over the SHA of the complaining email address, and the provider can compare it to the SHA hashes of their own. Or... they're going to have to turn the list (again, preferably with the entries hashed) over to you - and then, I suppose, you'll have to trust that they're giving you a truthful list.

But, either way, I don't see how the mere act of hashing the list is going to significantly alter the problems of nuisance complaints or of dealing with large lists. Hashing is a very cheap thing to do after all.