Nowadays, the widespread e-mail viruses and worms have caused large damage to many organizations and personal users. More and more users have realized how important and vulnerable e-mail is. Most organizations have implemented firewall and anti-virus software to try to protect their e-mail systems and networks. However, it is not a comprehensive defense and not enough. In this thesis, we will focus on e-mail safety, outline both possible threats to e-mail systems and defenses against the threats, and introduce a defense strategy and an SMTP filter system, a system which has different features compared with either traditional firewalls or traditional anti-virus software and can be a practical filter system to defend against e-mail threats.