I have checked several references on usage and style regarding this issue (Bryan Garner, James Kilpatrick, Morton Freeman, and several dictionaries). None were definitive. AO gave an example and asked "What's wrong with that?" My answer: it is an uncommon form of usage and is stilted, bulky, multisyllabic, and will cause readers to balk and/or re-read the sentence. You should not be writing in a manner that causes those reactions in your readers. While "correspondences" may not be technically incorrect (although I personally agree with the many contributors who said the noun is both singular and plural [it is perfectly acceptable to say or write, eg.: "How many different fish are there?]), it should be avoided. If that word does not fit your sentence structure, change the structure to accommodate the use of "correspondence" or use a different noun, e.g.: communications, letters, notes, emails, or other, more precise, (and plural) noun. Sorry I'm so late to your party, here. The subject just arose with my assistant and I was struggling to find authority to back up my preferred usage.