It all depends on the content of your most recent correspondence.
> If the other person has answered an important question that you had asked previously, you can use []{} as means "an answer to a question".
I used the word "important" because something like "How are you?" is not one of the questions I am talking about here.
The honorific would be a nice touch most of the time, but it would be optional if you were way "above" the other person.
> If no Q&A has been performed recently, you cannot use the word like above. You can say, instead, []{} as only means "reply".
That honorific is used virtually 100% of the time in this set phrase regardless of your relationship with the other person.