When I received The Will of Kyros missive, there were several response options, and I was able to view each one before deciding to send it by selecting Go Back after having selected the response. When clicking Go Back on The Dawn of Ascension missive, the selected reply is instead sent, and the missive is archived. I thought at first I must have mis-clicked, but this just happened on another playthrough, where I very carefully clicked the correct text. Since I was playing Trial of Iron, two of my characters have now irrevocably sent letters to their boss saying things I really didn't intend to say.