问题：执行 systemctl start postfix
Job for postfix.service failed because the control process exited with error code. See "systemctl status postfix.service" and "journalctl -xe" for details.
解决方法
vim /etc/postfix/main.cf

myhostname = mail.flycoder.cn
mydomain = flycoder.cn
myorigin = $mydomain
inet_interfaces = localhost // all表示接受来自所有网络的请求
mydestination = $myhostname, localhost.$mydomain, localhost  //指定发给本地邮件的域名
inet_protocols = ipv4