The MMP includes an SMTP proxy which is disabled by default. Most sites do not need the SMTP proxy because Internet Mail standards already provide an adequate mechanism for horizontal scalability of SMTP (DNS MX records).
The SMTP proxy is useful for the security features it provides. First, the SMTP proxy is integrated with the POP proxy to implement the POP before SMTP authorization facility required by some legacy POP clients. For more information, see Using the MMP SMTP Proxy in Sun Java Communications Suite 5 Deployment Planning Guide and 23.8 Enabling POP Before SMTP. In addition, an investment in SSL acceleration hardware can be maximized by using the SMTP proxy. See 23.5.4 How to Optimize SSL Performance Using the SMTP Proxy.