Now-a-days, network security has become very important. For those securities Simple Mail Transport Protocol is the most widely used protocol for e-mail delivery. But, it lacks security features like privacy, authentication and integrity of e-mail message. To make e-mail communication secure and private e-mail servers adopted one or more security features. The security protocols provide a most security but it also has several limitations. This paper discusses limitations of e-mail security protocols, analyses and evaluates their effectiveness in e-mail servers, as well as outlines the various attack methods which are used, and various defense mechanisms against them.