I'm currently sending NO, that hasn't caused any problems at least with
Evolution and OSX's Mail.app.
Actually when I think about it, I'm not sure about that. The condition is so rare that I can't be sure it has ever happened. It will probably show up only with busy shared mailboxes.
What's your exact condition for sending NO? That the message has been deleted by someone else?
Expunged, yes. Just like RFC2180 described it.
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