>I believe that my answer was correct in general, but there is always such thing as personal perception. IMO (and until now I believed it is the case) the initiator should be the only person to mark messages.

That's essentially true. However, what would you do with the cases when it obviously was a solution (based on the response, something like "Thank you so much, works like a treat"), yet it is not marked? In such cases I think I can do it, especially for the older threads when there is no chance the originator will ever return.

It doesn't apply for this case, though, I agree.