Is it okay to address "Prof. X" as "Mr. X"? I am from the Netherlands.

I have seen quite a few threads which address the question of whether or not it is acceptable to address a professor by their first name. However, none of them seem to address the point of whether it is still permissible to refer to them as Mr/Mrs X?

For example, if you know that someone's full title is "Prof. Dr. X" is it acceptable to write them an email starting with "Dear Mr/Mrs X"?
This will depend on the norm within your country to some extent, but I'm inclined to say that at least in the US, calling a professor 'Mr.' is insulting, provided you are in an academic context. 

My neighbors, many friends, and the cashier at the cafe know me as Mr. X. But on campus, I'm Dr. X...and you know I'm a professor. Using 'Mr.' seems strangely intentional in a way to diminish ones accomplishments.