使用 Xinetd 端口代理

使用 Xinetd 端口代理 Gmail 的 SMTP：
此法针对客户端，Web 端请自己想办法
yum install -y xinetd

cat /etc/xinetd.d/qqmail

service smtp-465
{
type = UNLISTED
port = 465
bind = 0.0.0.0
socket_type = stream
wait = no
user = nobody
redirect = smtp.exmail.qq.com 465
per_source = UNLIMITED
cps = 100 2
}
service smtp-587
{
type = UNLISTED
port = 587
bind = 0.0.0.0
socket_type = stream
wait = no
user = nobody
redirect = smtp.exmail.qq.com 587
per_source = UNLIMITED
cps = 100 2
}

修改本地host指向代理即可通过客户端发邮件：
xxx.xxx.xxx.xxx smtp.exmail.qq.com