A SMTP server can send mail from local machine by default.
It also can relay mail from other machine/device if relay function is enabled.
SMTP 伺服器預設只可以允許本機送出郵件。
如果開啟轉發功能的話，就可以讓外部機器經由本機送出郵件。
OS: CentOS 5.3
作業系統: CentOS 5.3
Reference
http://apt.nc.hcc.edu.tw/web/student_server_centos/student_server_centos.html#smtp
