Verbs that refer to a specific action. For example I will keep in contact with you, this month. The precise verb for that sentence would be I will email you, this month.
I think the single sentence that shows it is a personal letter is "that all I can say is that I am well" as the rest of the letter is details about his military campaign so could be part of a more formal memo if it were not for this opening sentence so that seems to be how to identify it as such.
