Rhel7本地邮件系统，服务器和客户机都配置postfix和null client
本地邮件：loopback-only,目地无，中继他给，网络自己，源域名，本地转发禁用
Vim /etc/postfix/main.cf
Inet_interfaces=loopback-only
Mydestination=
Relayhost=[smtp.desktopX.example.com]
Myorigin=desktopX.example.com
Mynetworks=127.0.0.0/8[::1]/128
Local_transport=error:local delivery diskabled
systemctl restart postfix.service
systemctl enable postfix