All in Markdown.
We 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 TErminaL NETwork 或 TELetype NETwork 或 TELecommunication NETwork ，是一个流行于Unix系统的网络协议，直接由终端操作。Telnet被发明于1969年，安全性并不在其设计者的考量范围内，因此其安全性极低，它正在被SSL等更安全保密的网络协议所取代。
准备工作
使用 Homebrew 安装 Telnet：
打开终端，输入下列命令安装 Homebrew ；参考
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
安装 Telnet
brew install telnet
之后就可以在终端使用 Telnet 了。
借助 Telnet 通过 POP3 接收PKU邮箱中的一封邮件
首先，用Outlook邮箱向 pku mail 发送一封邮件。然后，在pku邮箱设置-高級功能选项卡处查询到 pkumail 的 POP3 端口为 110；
打开终端，输入telnet mail.pku.edu.cn 110
MacBuch-Pro:~ usr$ 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])
#终端返回的内容显示我们已经连接到 coremail（pku mail）所属 POP3 server
#然后就可以输入用户名和密码登录邮箱
#注意，如果持续一段时间未操作，连接将被关闭。待光标出现时即可输入后续内容。
user goat #输入用户名
+OK core mail
pass woshimima #輸入密碼
+OK 982 message(s) [336623586 byte(s)]
#显示登录成功，邮箱内有982封邮件（所占空间 336623586 bytes），其中第982封邮件就是刚刚发送的测试邮件。
list #輸入list按順序查看每封邮件及其所占的空间
+OK 982 336623586
1 7972
2 3071
3 13886
4 13625
…… #由於郵件數量龐大，只截取一部分
978 6089
979 3234
980 5094
981 7768
982 7830
.
retr 982 #檢索（retrieve）第982封郵件
輸出結果
+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)
ARC-Seal: i=1; a=rsa-sha256; s=arcselector9901; d=microsoft.com; cv=none;
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ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com;
s=arcselector9901;
h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1;
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ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=none; dmarc=none;
dkim=none; arc=none
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=outlook.com;
s=selector1;
h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
bh=JdssuO+CI3iRJ/jiZ/dFi1y4a3fBqn1ksevuq6B/ZHQ=;
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Received: from TYCP286MB2003.JPNP286.PROD.OUTLOOK.COM (2603:1096:400:15b::12)
by TYBP286MB0221.JPNP286.PROD.OUTLOOK.COM (2603:1096:404:8027::16) with
Microsoft SMTP Server (version=TLS1_2,
cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.5417.17; Fri, 8 Jul
2022 06:21:56 +0000
Received: from TYCP286MB2003.JPNP286.PROD.OUTLOOK.COM
([fe80::5ced:6265:1b9d:15f3]) by TYCP286MB2003.JPNP286.PROD.OUTLOOK.COM
([fe80::5ced:6265:1b9d:15f3%6]) with mapi id 15.20.5395.022; Fri, 8 Jul 2022
06:21:56 +0000
From: Chien <juen.chien@outlook.com>
To: "goat@pku.edu.cn" <goat@pku.edu.cn>
Subject: Test Mail
Thread-Topic: Test Mail
Thread-Index: AQHYkpMFN1SV7u59+0CWHMpXZc0VtA==
Date: Fri, 8 Jul 2022 06:21:56 +0000
Message-ID: <F7E722BB-C004-406D-98F3-F76FB576A4B2@outlook.com>
Accept-Language: en-GB, zh-CN, en-US
Content-Language: en-US
X-MS-Has-Attach:
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x-ms-exchange-messagesentrepresentingtype: 1
x-tmn: [ZUaxuFH/Z8N17vzKFFUDhIoT6I5KR+sq]
x-ms-publictraffictype: Email
x-ms-office365-filtering-correlation-id: 60c3299d-c139-4f69-7927-08da60aa2824
x-ms-traffictypediagnostic: TYBP286MB0221:EE_
x-microsoft-antispam: BCL:0;
x-microsoft-antispam-message-info:
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Dear Goat
Hi, this is an Email for testing telnet.=20
Have a nice day.
Sincerely
Dog
.
輸出結果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/