A new technique for data hiding in emails via Outlook Express and IE is proposed. Secret data are encoded by special ASCII control codes and embed-ded into cover emails by inserting the data into email text line ends. These special codes were found out by a systematic test of all the ASCII codes on various email server software systems and standards. They are invisible to the user when displayed, achieving the effect of steganography. The proposed data en-coding technique is a combination of five coding rules found in this study. The inserted codes will not change the meanings of the sentences in the cover email, neither causing any noticeable difference to the reader. Furthermore, hidden data can be extracted from a stego-email completely to recover the original email text content. Also described are two applica-tions of the proposed data hiding technique, namely, covert communication via emails and authentication of emails. In the former application, security is en-hanced by the use of a secret key, and in the latter, an authentication signal is generated from the cover email for email fidelity checking. Good experimental results show the feasibility of the proposed method.