Negative bias is the term used by psychologists to describe a natural human feeling when reading e-mails. In practice it means if a sender feels positive about a message, the recipient feels neutral. Neutrality on the sender's side, however, means negativity on the part of the recipient.
The reason is simple: the lack of nonverbal cues involved in face to face communication. From the text the recipient can't recognise the sender's facial expression, what gestures were used or whether they raised their voice while creating the message. Thus the recipient automatically assumes the worse option.
