All in Markdown. I use PKU Mail as an instance.
Manually Receive an Email with Telnet under POP3 (on macOS)
使用 Telnet 手动接收邮件 （基于 macOS 系统）
什么是 Telnet
Telnet 代表 rminawork 或 etype work 或 ecommunication work ，流行Unix终端直接操作系统的网络协议。Telnet1969年发明的安全性不在设计师的考虑范围内，所以安全性极低，正在被发明SSL更安全保密的网络协议被取代。
准备工作
使用 Homebrew 安装 Telnet： 打开终端，输入以下命令安装 Homebrew ；参考
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)" 安装 Telnet
brew install telnet 之后就可以在终端上使用了 Telnet 了。
借助 Telnet 通过 POP3 接收PKU邮箱里的邮件
首先，用Outlook邮箱(或任何其他邮箱) pku mail 发送测试邮件。然后在邮箱设置-高级功能选项卡处查询 pkumail 的 POP3 端口为 110；
打开终端，输入telnet mail.pku.edu.cn 110
MacBook-Pro:~ usr$ telnet mail.pku.edu.cn 110 Trying 162.105.129.99... Connected to mail.lb.pku.edu.cn. Escape character is '^]'. OK Welcome to coremail Mail Pop3 Server (mispb-1ea67e80-64e4-49d5-bd9f-3beeae24b9f2-pku.edu.cns[7f9fc2345ba1662f5ae8c6faa3cd1742s]) ##终端返回显示我们已经连接到了 coremail（pku mail）所属 POP3 server 然后输入用户名和密码登录电子邮件 #请注意，如果连接不工作一段时间，连接将关闭。当光标出现时，可以输入后续内容。 user goat #输入用户名 OK core mail pass woshimima #输入密码 OK 982 message(s) [336623586 byte(s)] #显示登录成功，邮箱内有982封邮件(占用空间) 336623586 bytes），第982封邮件是刚刚发送的测试邮件。 list #输入list按顺序查看每封邮件及其空间 OK 982 336623586 1 7972 2 3071 3 13886 4 13625 …… ##由于邮件数量庞大，只截取了一部分 978 6089 979 3234 980 5094 981 7768 982 7830 . retr 982 #检索（retrieve）第982封邮件 输出结果
OK 7830 octets Received: from JPN01-TYC-obe.outbound.protection.outlook.com (unknown [40.92.99.65]) by front01 (Coremail) with SMTP id 5oFpogCHpiQGzcdinU9fAA--.11299S3; Fri, 08 Jul 2022 14:21:59 +0800 (CST)
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From: Chien <juen.chien@outlook.com>
To: "goat@pku.edu.cn" <goat@pku.edu.cn>
Subject: Test Mail
Thread-Topic: Test Mail
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Date: Fri, 8 Jul 2022 06:21:56 +0000
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Dear Goat
Hi, this is an Email for testing telnet.
Have a nice day.
Sincerely
Dog
.
輸出結果的最後是那封測試郵件的內容，此外還包括發送者和接受者的一些信息。
刪除和退出
...
dele 977 #可删除第977封邮件
+OK core mail
quit #可关闭连接
+OK core mail
Connection closed by foreign host.
参考文献
主要参考： https://www.lampdocs.com/working-with-e-mail-using-telnet/ https://www.vircom.com/blog/quick-guide-of-pop3-command-line-to-type-in-telnet/
辅助参考： https://www.minitool.com/lib/what-is-telnet.html https://www.geeksforgeeks.org/introduction-to-telnet/ https://www.scalahosting.com/kb/how-to-connect-to-telnet-via-mac-terminal/ https://osxdaily.com/2018/07/18/get-telnet-macos/ https://www.siteground.com/tutorials/email/protocols-pop3-smtp-imap/