参考文章

https://www.zoho.com/mail/help/how-email-works.html

https://www.cnblogs.com/ysocean/p/7652934.html

https://www.cnblogs.com/ysocean/p/7653252.html

https://www.cnblogs.com/ysocean/p/7663695.html

https://www.cnblogs.com/ysocean/p/7666061.html