操作流程

telnet 打开一个 TCP 连接

$ telnet pop.qiye.aliyun.com 110             
                           
Trying 140.205.77.6...
Connected to pop.qiye.aliyun.com.
Escape character is '^]'.
+OK AliYun POP3 v0.1.1 server ready

+OK 表示连接成功

输入用户名

USER xxx@xxxx.com 

+OK xxxxx

USER + 空格 + 自己的企业邮箱

+OK 表示成功

输入密码

PASS xxxx

+OK 220 message(s) [12808929 byte(s)]

PASS + 空格 + 自己的企业邮箱密码

输入查询命令

LIST 

+OK 220 12808929
1 1788
2 7894
3 1794
4 34424
5 9004
6 16701
7 16700
8 16700
9 622821
10 3066
...

LIST 可以查询邮箱有哪些邮件，前面的是邮件编号，后面是大小，是按照邮箱收件的顺序展示的。

查看邮件

RETR 1
+OK 1788 octets
X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R671e1;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018047212;MF=postmaster@xxx.com;NM=1;PH=DW;RN=1;SR=0;TI=W4_admin1_0AC26404_1617759830154_c116c;
Received: from WS-web (postmaster@xxx.com[W4_admin1_0AC26404_1617759830154_c116c]) by ay29a011140100061.et135 at Wed, 07 Apr 2021 09:43:48 +0800
Date: Wed, 07 Apr 2021 09:43:48 +0800
From: "postmaster" <postmaster@xxx.com>
Return-Path: "postmaster" <postmaster@xxx.com>
To:  <xxx@xxx.com>
Reply-To: "postmaster" <postmaster@xxx.com>
Message-ID: <ad489201-2015-4031-95c8-24193ad83718.postmaster@xxx.com>
Subject: =?UTF-8?B?5oKo5bey57uP5Yqg5YWl6YKu5Lu257uEW+WFqOS9k+aIkOWRmF0oYWxsQGRhdGFncmFuZC5j?=
  =?UTF-8?B?b20p?=
X-Mailer: [Alimail-Mailagent revision -899287049]
MIME-Version: 1.0
Return-Path: <postmaster@xxx.com>
Content-Type: multipart/alternative;
  boundary="----=ALIBOUNDARY_10298_7fdbeda9c700_606d0e54_a75216"

------=ALIBOUNDARY_10298_7fdbeda9c700_606d0e54_a75216
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: base64

CgogICAgICAgICAgICDmgqjlpb3vvJoKCiAgICAgICAg5oKo5bey6KKr5Yqg5YWl6YKu5Lu257uE
44CCCgogICAgCg==
------=ALIBOUNDARY_10298_7fdbeda9c700_606d0e54_a75216
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: base64

PGRpdiBjbGFzcz0iX19hbGl5dW5fZW1haWxfYm9keV9ibG9jayI+PGRpdiBzdHlsZT0icGFkZGlu
ZzowO21hcmdpbjogMDtib3JkZXI6IDA7Zm9udC1zaXplOiAxMDAlIj4KCiAgICAgICAgICAgIDxw
IHN0eWxlPSJtYXJnaW46MDtwYWRkaW5nOjA7Ij7mgqjlpb3vvJo8L3A+CgogICAgICAgIDxwIHN0
eWxlPSJtYXJnaW46MDtwYWRkaW5nOjA7dGV4dC1pbmRlbnQ6MmVtOyI+5oKo5bey6KKr5Yqg5YWl
6YKu5Lu257uE44CCPC9wPgoKICAgIDwvZGl2Pgo8L2Rpdj4=
------=ALIBOUNDARY_10298_7fdbeda9c700_606d0e54_a75216--

.

RETR + 空格 + 邮件编号，可以查看邮件。

邮件的正文是以空行和一个点结束，然后每一段正文又被 base64 编码了，multipart/alternative 协议定义了下面有两段内容一段是纯文本(text/plain)和一段是超文本(text/html)的，可以分别解开查看，内容是一致的，超文本里面会有 html 标签的信息，可以展示丰富的显示效果。