Here is a simple question from a person with a limited knowledge in German.
Currently I'm writing a letter to a person who has a title of "Graf zu" someplace. Now how would you properly address him? Dear Sir or Dear Graf zu ...?
In German you traditionally wrote "Sehr geehrter Herr Graf zu XXX,...".
Nowadays you can simply write "Sehr geehrter Graf zu XXX,...". And only a chauffeur or butler are supposed to use "Herr Graf" without the name. Nobility was abolished by the Weimar Republic in 1919. No new titles were to be awarded and existing titles were to be considered as part of the name.
PS: Actually your question is about English and might be asked on English Language and Usage (http://english.stackexchange.com). You might start your letter according to the German rules above as a sign of respect and continue in English...