220 Welcome message ...
输入HELP可以得到一些帮助信息：
help
214-This is Sendmail version 8.9.2
214-Topics:
214- HELO EHLO MAIL RCPT DATA
214- RSET NOOP QUIT HELP VRFY
214- EXPN VERB ETRN DSN
214-For more info use "HELP <topic>".
214-To report bugs in the implementation send email to
214- sendmail-bugs@sendmail.org.
214-For local information send email to Postmaster at your site.
214 End of HELP info
可以看到这是一个流行的SMTP服务软件Sendmail，版本是8.9.2。
通常的发送邮件命令顺序为：
MAIL FROM:"Dallas Thunder"<d.t@126.com>
250 "Dallas Thunder"<d.t@126.com>... Sender ok
RCPT TO:"Dallas Thunder"<d.t@126.com>
250 "Dallas Thunder"<d.t@126.com>... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
subject:First hello
Hi, this is a test.
.
250 OAA06479 Message accepted for delivery
这封邮件就已经排队待发了，对用户来说等于已经发出。至于连接到端口110就更可气了，我用的这个居然连help也不支持，只能靠瞎猜得到了这么一些用法。连通后先是欢迎信息：
+OK Welcome message...
然后输入用户名和密码进入：
USER dallas
+OK Password required for dallas.
PASS password
+OK dallas has 2 messages (2961 octets).
LIST
+OK 2 messages (2961 octets)
1 1338
2 1623
.
怎么列出邮件的Subject我可不知道。读邮件输入：
RETR 1
+OK Message follows
（然后是邮件的源代码，MIME信息需要e-mail客户软件来处理，开头是一堆路由信息）
Received: from smtp.szonline.net (webmail.szonline.net [202.96.154.33])
by szonline.net (8.9.2/8.9.2) with ESMTP id PAA21264
for <dallas@szonline.net>; Wed, 9 Jun 1999 15:05:17 +0800 (CST )
Received: from public1.szptt.net.cn (public1.szptt.net.cn [202.96.134. 166])
by smtp.szonline.net (8.9.3/8.9.3) with ESMTP id PAA01629;
Wed, 9 Jun 1999 15:04:39 +0800 (CST)
Received: from SZ-SVR-2.sz.gnet.gd.cn (SZ-SVR-2.sz.gnet.gd.cn [172.21. 16.86])
by public1.szptt.net.cn (8.9.2/8.9.2) with ESMTP id PAA22717;
Wed, 9 Jun 1999 15:05:02 +0800 (CST)
Received: from b0505a ([172.21.79.28])
by SZ-SVR-2.sz.gnet.gd.cn (8.9.2/8.9.2) with SMTP id PAA10622;
Wed, 9 Jun 1999 15:02:50 +0800 (CST)
Message-ID: <004801beb246$f0396260$4400080a@b0505a>
From: "Dallas Thunder" <d.t@126.com>
To: "Dallas Thunder"<d.t@126.com>
Subject: First hello
Date: Wed, 9 Jun 1999 15:08:55 +0800
MIME-Version: 1.0
X-Priority: 3
X-MSMail-Priority: Normal
X-Mailer: Microsoft Outlook Express 5.00.2014.211
X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2014.211
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from base64 to 8bit by szonline.net id PAA21264
Content-Type: text/plain;
charset="gb2312"
X-UIDL: ‘1)e9pe-e9WP`!!T4%e9
Status: U
Hi, this is a test.
如果从服务器中删除邮件，则输入：
DELE 1
+OK Message 1 has been deleted.
e-mail的客户程序通常都有一个选项“将邮件的副本留在服务器”，如果没有选中这一项那么客户程序会在接收完所有邮件后执行DELE命令，如果选中则不执行。
最后是退出：
QUIT
+OK Pop server at szonline.net signing off.
可见发送邮件不需要身份验证，为了防止垃圾邮件泛滥，现在很多的SMTP服务器都采取了限制收发一方的手段，也就是说收或发的至少一方必须是本网用户才可以发送邮件，否则会得到一个550或者553的出错信息。
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