A comma is often used just to indicate a pause in speaking. Adding the comma would depend on the writer's speaking style. In other words, in informal writing both would be correct. In a more formal written style the second option would be better - a prepositional phrase is not a new clause, therefore doesn't require a comma.

The first with comma, 100% correct and it fits the requirement for academic English convention. (If you are taking any ACT/SAT test, you have to follow such rules to get a high score.) Why comma? Because “in your letter” is already an adverbial phrase, and after it goes the main sentence, so main sentence needs to be separate to avoid confusion or ambiguity. The second one without is all right in spoken or informal writing.

A comma is usual when you introduce a main clause with a prepositional or adverbial phrase. I would not say that it was essential, except if its absence created an ambiguity. There is no ambiguity in your example and so there is no problem. However, I recommend using a comma as ambiguities are less obvious to students than to native speakers.