第二步：转发主机到目的主机
R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIE.ARPA
R: 250 BBN-VAX.ARPA
S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
R: 250 OK
S: RCPT TO:<Jones@BBN-VAX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
2 Nov 81 22:40:10 UT
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <JQP@MIT-AI.ARPA>
S: Subject: The Next Meeting of the Board
S: To: Jones@BBN-Vax.ARPA
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
确认和发送
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
S: SEND FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
获得和发送邮件 首先确定用户名，然后尝试将邮件发送到用户终端，
当它失败时，发送到用户邮箱。
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
S: SEND FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 450 User not active now
S: RSET
R: 250 OK
S: MAIL FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
上述问题的更有效的作法
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
S: SOML FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 User not active now, so will do mail.
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
邮件列表 首先，两个邮件列表中的每一个在不同主机的不同会话上扩展，
然后，通过转发主机向列表上的用户发送邮件。
第一步：扩展第一个列表
R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-AI.ARPA
S: EXPN Example-People
R: 250-<ABC@MIT-MC.ARPA>
R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<joe@foo-unix.ARPA>
R: 250 <xyz@bar-unix.ARPA>
S: QUIT
R: 221 MIT-AI.ARPA Service closing transmission channel
第二步：扩展第二个列表
R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-MC.ARPA
S: EXPN Interested-Parties
R: 250-Al Calico <ABC@MIT-MC.ARPA>
R: 250-<XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<fred@BBN-UNIX.ARPA>
R: 250 <xyz@bar-unix.ARPA>
S: QUIT
R: 221 MIT-MC.ARPA Service closing transmission channel
第三步：通过转发主机向包括于两个列表中的所有用户发送邮件
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 USC-ISIE.ARPA
S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
R: 250 OK
S: RCPT
TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
转发的情况
R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISIF.ARPA
S: MAIL FROM:<mo@LBL-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF.ARPA>
R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIF.ARPA Service closing transmission channel
第一步：尝试第一台主机上的邮箱
R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISIF.ARPA
S: MAIL FROM:<mo@LBL-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF.ARPA>
R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
S: RSET
R: 250 OK
S: QUIT
R: 221 USC-ISIF.ARPA Service closing transmission channel
第二步：尝试第二台主机上的邮箱
R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISI.ARPA
S: MAIL FROM:<mo@LBL-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<Jones@USC-ISI.ARPA>
R: OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISI.ARPA Service closing transmission channel
许多接收者的情况
R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BERKELEY.ARPA
S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<fabry@BERKELEY.ARPA>
R: 250 OK
S: RCPT TO:<eric@BERKELEY.ARPA>
R: 552 Recipient storage full, try again in another transaction
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<eric@BERKELEY.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BERKELEY.ARPA Service closing transmission channel
名词表:
<CRLF>
回车
<SP>
空格
ASCII
美国标准信息交换码
主机
拥有SMTP进程或邮箱的网络计算机
发送SMTP进程
与接收SMTP进程一起工作的进程。发送SMTP开始传输服务连接，它发出
SMTP命令，接收应答，管理邮件的传送
用户
希望获得邮件服务的人（或以人的名义出现的进程），还有就是邮件的接收者。
会话
当传输信道打开时进行的一系列信息交换
传输服务
可靠的面向流的数据通信服务。例如：NCP，TCP，NITS。
传输信道
在发送SMTP和接收SMTP之间建立的全双工的用于交换命令，应答和邮件内容的信道
字符
可显示字符串
行
以一个<CRLF>结束的邮件内容
应答
接收SMTP对发送SMTP的通过传输信道发送的的对某一命令的（成功或不成功的）
响应。应答的一般格式是应答码加一段文本。通常情况下，应答码供机器使用，
而文本用于人类用户使用
邮件内容
一系列的字符串，它们符合ARPA Internet文本信息格式标准的标准字符集
邮件内容结束标记
标明邮件内容结束的特定字符
邮箱
指定应该向何处发向用户的信件的地址（字符串）。它通常由用户名和主机名表示
命令
由发送SMTP发送到接收SMTP的要求一个邮件服务操作的请求
域
邮件系统中主机地址字符串的层次式表示
接收SMTP进程
与发送SMTP进程一起工作的进程。它等待通过传输服务建立的连接。它接收发送
SMTP发出的命令，给出应答并执行相应的操作
操作
一个信息由一个接收者发送到另一个或多个接收者的一系列操作 ,