SMTP 和 POP3 概念？
根据下图取自 tcp/ip 协议套件...如果只有一台计算机连接到邮件服务器，则不需要 SMTP 和 POP3/IMAP4！
如果这是真的，那么它是否可以使用 telnet 建立 tcp 连接，然后呢？
According to the below figures taken from an tcp/ip protocol suit... SMTP AND POP3/IMAP4 are not required if there is one and only one computer attached to the mail server!
If that's true then have does it work make a tcp connection by using telnet and then what?
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你的问题不太清楚，我不太明白你从哪里得出这个结论。
不过，我会根据我所看到的情况做出一些假设。
如果所有内容都在一台服务器上，那么使用电子邮件的两方只需使用 telnet 或远程桌面或其他方式登录到服务器，然后直接从邮件服务器检索电子邮件。例如，在unix时代，你只需输入“mail”，然后你的电子邮件就会有一个真正可怕的命令行界面——它不使用pop3或任何东西。我不确定是否有人真的会再这样做，并且在当今的大多数系统上，甚至本地邮件访问也是通过 pop3 或 imap 完成的，只是为了保持一切标准。
如果您的客户端是远程的——即图 20.1 中的两个 UA 位于工作站上，那么它们肯定会使用 POP3 或 IMAP 或等效协议。
SMTP 主要用作服务器之间的传输，因此如果您的两方位于不同的邮件服务器上，那么您可以使用 SMTP 在这两个服务器之间获取邮件。这不是唯一的方法，例如，您可以使用 POP3 检索器，尽管标准肯定是使用 SMTP 并将两台服务器设置为 MTA。
我不太明白您所展示的内容如何得出不需要 POP3/IMAP 的结论 - 当然，如果只有一台服务器涉及它们，那么 SMTP 肯定不是，但通常仍然会使用 POP3 或 IMAP发生。
当然，除了 POP3 和 IMAP 之外，还有其他检索电子邮件的方法，例如 Exchange 的方法。
总而言之，您展示的幻灯片本身看起来非常好，并且可能脱离上下文，但我不知道您从哪里得出结论。
Your question isn't very clear, I can't quite see where you're drawing this conclusion from.
However, i'll make some assumptions based on what I can see.
If everything is on one server then the two parties using email can simply log in to the server using telnet or remote desktop or something and retrieve email directly from the mail server. E.g. in unix days you'd just type "mail" and have a truly horrendous command line interface to your email - which did NOT use pop3 or anything. I'm not sure if anyone would really do anything this way any more, and on most systems these days even local mail access is done via pop3 or imap just to keep everything standard.
If your clients are remote - i.e. the two UA's in question in figure 20.1 are on the workstations, then they would certainly be using POP3 or IMAP or equivalent.
SMTP is used primarily as a transport between the servers, so if your two parties are on different mail servers then you might use SMTP to get the mail between those two servers. That's not the only way, you could use POP3 retrievers for example, though the norm certainly would be to use SMTP and set the two servers up as MTA's.
I don't quite follow from what you've shown how you've drawn the conclusion that POP3/IMAP are not required - certainly if there's only one server involved them SMTP certainly isn't but the use of POP3 or IMAP would normally still take place.
Of course there are other methods of retrieving email besides POP3 and IMAP such as the way that Exchange does it.
To sum up, the slides you've shown look perfectly fine on their own and potentially out of context, but I have no idea where you've drawn your conclusion from.