etiquette - 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"?