The point is though enrico, is that there is no need to do it at all....
If you disagree with something that someone says, then by all means post your opinion and back it up with reason and facts.... no one would find that demeaning..
The fact is that people join the forum to express opinions and get advise....not be made to feel stupid...
if someone thought the op was incorrect (which we all are sometimes) and felt the need to PM them about it, fine, but it should have been a case of "I think you might be incorrect, and this is why....", not a nasty PM like the OP appears to have received.