Every patient in any state hospital for the mentally ill shall be allowed to write whenever the patient desires and to whomever the patient may choose, and the chief executive officer of the hospital, upon request, shall supply each patient, not otherwise supplied, with suitable writing materials and postage, at the expense of the state, sufficient for writing at least one letter per week. Such letters shall be regularly and promptly collected and shall be placed in the United States mail for delivery unless the chief executive officer of the hospital has on file a written request from a recipient or potential recipient that letters shall not be mailed to such recipient or unless there is reasonable cause to believe that the contents of any letter are threatening.