The initial phrase - "Dear policeman" - first of all, most American users would use plural ("policemen" instead of "policeman"), and most people would use slang ("cops" instead of "policeman") - since slang is difficult for foreign speakers to accomodate, I would give high odds that the person who wrote this is not a native-born American, but a recent arrival.
Dear Policeman is just odd phrasing for any American adult to write or say....It would be maybe Dear Policemen (maybe)
Dear Cops, or Hey Cops etc were more likely if you were taunting them.
The only time you would maybe be specific like Dear Mailman (because it's likely to be one specific person)