ACLs for "black hole" folder?

[Andreas Hasenack]

I'm trying to set up two spam related folders: one called "spam" and the
other called "nospam". Both are shared folders and the intention is that
users put there mail that was incorrectly marked as spam or ham.

The "nospam" folder in particular whould have to be a "black hole":
users can put messages there but not read them back or list them,
because these are potentially private messages and are not spam.

For now I used these ACLs: lip. Seems to work: users can post and insert
new messages there, but not see them.

The problem is that some mail clients (kmail and evolution so far) popup
a big fat warning about not being able to read that folder, and this
happens every time the user checks for new mail. It's annoying.

Is there another ACLs that could be used that would prevent this from
happening? Something to make the folder look like it's empty, for
example?

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