Hightened emotion in general can lead to a low clarity of thought. Emotions can cloud the decision making process leading to some really bad spur of the moment decisions, especially if you are depressed or angry. Writing in a high state of emotion can be a source of great inspiration or the pit from which you dredge up things you wish you had never written.
About to hash out an angry email, it is worth waiting a bit to calm down. Once you click the send button there is not a lot you can do to undo the damage.