My standard reply, unless it's something specific that can't be addressed on the forum, is just to remind them to ask their question in the open (and appropriate) section of the forums. Most of the time their simply forgetting that out here, they'll get replies from the community, not just me. And that the answer they get, will help others when searching with the same question. I also remind them that sometimes, no one knows the answer. But they're very likely not going to reply to your thread with the post "I don't know." That wouldn't be helpful.