linux使用mail发送外部smtp邮件
https://www.cnblogs.com/bjx2020/p/9591287.html
https://blog.51cto.com/8588192/2308876?source=dra
https://www.cnblogs.com/xiaoshi1991/archive/2012/09/19/2694465.html
https://www.cnblogs.com/findyou/p/5760970.html
https://blog.51cto.com/wuchunwei/1758685
https://www.xiaoz.me/archives/9862
Linux 命令（网络管理 - mail/mailx）
https://blog.csdn.net/liang19890820/article/details/53115334
https://www.jianshu.com/p/5185bf3f4d72?tdsourcetag=s_pctim_aiomsg
在 Linux 命令行发送邮件的 5 种方法(推荐)
https://www.jb51.net/article/176152.htm
linux下sendmail邮件系统安装操作
https://www.cnblogs.com/kevingrace/p/6143977.html
https://blog.csdn.net/bodybo/article/details/80817968
Linux下邮件服务postfix安装
https://blog.csdn.net/wj78080458/article/details/85247428
https://blog.csdn.net/cjju71217/article/details/100250807
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