Now, if you're someone's boss and hitting up a subordinate with whom there is known "bad blood" then I get the point about the "emotional cost" of a scenario where you write, close to the end of the business day, "hello", then your subordinate doesn't respond for 10 minutes. Then you go home, or get into a block of meetings that will make you unavailable to continue the convo for several hours.
But if you have that little emotional intelligence, you shouldn't be anyone's boss, and if a mere "hello" from your boss freaks you out, you should probably just quit then and there.