Therefore is reasonable in that case, though it still reads a bit clumsy. "The lawsuit was dropped" seems like the most important part of that blurb, so leading with it flows better. "The lawsuit was dropped after he didn't send the letter" is so much nicer. You get to the point and explain it immediately after instead of giving the reader information you have to contextualize after. "Therefore" just reads as pedantic and overbearing in most situations in my opinion (and I guess my teacher's opinion too).