Years ago when the company I worked for got email capability we had to take a class on email etiquette and the big take away from that was no one can see you.
When you talk with someone face to face, you words combined with your facial expressions and gestures tell the person you are talking to if you are happy, sad, angry etc. so your tone, tenor and words are not the total message. How ever with email/messages you don't have the luxury of "smoothing" out your tone and tenor with your physical presence so you have to be aware that the humor in your head is not necessarily coming thru your message.