1. sendmail配置/etc/hosts为：
127.0.0.1 aaa.com localhost aaa #aaa是主机名前缀
2./etc/hostname:
aaa.com
3./etc/mail/local-host-names
aaa.com
localhost
4.service sendmail restart

e.g.
echo "Hello World!" | mail -s "Test Title" root@localhost #发送Hello World给本机root用户

echo "Hello 163" | mail -s "This is title" xxx@163.com #发送给163邮箱用户（云主机测试成功，内网树莓派没成功，未解）