Exchanging emails is not only one of the most important tools in daily business but also in social life. Often information of high value and confidentiality is exchanged via emails. On the other hand the SMTP protocol used for exchanging emails is one of the earliest developed protocols without much focus on security, confidentiality or integrity.
Today the majority of the emails traveling through the Internet are considered Spam. Besides the annoying nature of Spam mail and the productivity loss for companies as employees get distracted; the significant risk imposed from spam is when it is used as a transport mechanism for Viruses and Trojans to propagate or when it is used to gain confidential information from employees by lurking them to infected websites.