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

轉發的情況

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：嘗試第一臺主機上的郵箱

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：嘗試第二臺主機上的郵箱

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

許多接收者的情況

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

名詞表:

<CRLF>
回車

<SP>
空格

ASCII
美國標準信息交換碼

主機
擁有SMTP進程或郵箱的網絡計算機

發送SMTP進程
與接收SMTP進程一起工作的進程。發送SMTP開始傳輸服務連接，它發出SMTP命令，接收應答，管理郵件的傳送

用戶
希望獲得郵件服務的人（或以人的名義出現的進程），還有就是郵件的接收者。

會話
當傳輸信道打開時進行的一系列信息交換

傳輸服務
可靠的面向流的數據通信服務。例如：NCP，TCP，NITS。

傳輸信道
在發送SMTP和接收SMTP之間建立的全雙工的用于交換命令，應答和郵件內容的信道

字符
可顯示字符串

行
以一個<CRLF>結束的郵件內容

應答
接收SMTP對發送SMTP的通過傳輸信道發送的的對某一命令的（成功或不成功的）響應。應答的一般格式是應答碼加一段文本。通常情況下，應答碼供機器使用，而文本用于人類用戶使用

郵件內容
一系列的字符串，它們符合ARPA Internet文本信息格式標準的標準字符集

郵件內容結束標記
標明郵件內容結束的特定字符

郵箱
指定應該向何處發向用戶的信件的地址（字符串）。它通常由用戶名和主機名表示

命令
由發送SMTP發送到接收SMTP的要求一個郵件服務操作的請求

域
郵件系統中主機地址字符串的層次式表示

接收SMTP進程
與發送SMTP進程一起工作的進程。它等待通過傳輸服務建立的連接。它接收發送SMTP發出的命令，給出應答并執行相應的操作

操作
一個信息由一個接收者發送到另一個或多個接收者的一系列操作
參考資料
   [1]  ASCII

      ASCII, "USA Code for Information Interchange", United States of
      America Standards Institute, X3.4, 1968.  Also in:  Feinler, E.
      and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
      the Defense Communications Agency by SRI International, Menlo
      Park, California, Revised January 1978.

   [2]  RFC 822

      Crocker, D., "Standard for the Format of ARPA Internet Text
      Messages," RFC 822, Department of Electrical Engineering,
      University of Delaware, August 1982.

   [3]  TCP

      Postel, J., ed., "Transmission Control Protocol - DARPA Internet
      Program Protocol Specification", RFC 793, USC/Information Sciences
      Institute, NTIS AD Number A111091, September 1981.  Also in:
      Feinler, E. and J. Postel, eds., "Internet Protocol Transition
      Workbook", SRI International, Menlo Park, California, March 1982.

   [4]  NCP

      McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
      January 1972.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
      Protocol Handbook", NIC 7104, for the Defense Communications
      Agency by SRI International, Menlo Park, California, Revised
      January 1978.

   [5]  Initial Connection Protocol

      Postel, J., "Official Initial Connection Protocol", NIC 7101,
      11 June 1971.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
      Protocol Handbook", NIC 7104, for the Defense Communications
      Agency by SRI International, Menlo Park, California, Revised
      January 1978.

   [6]  NITS

      PSS/SG3, "A Network Independent Transport Service", Study Group 3,
      The Post Office PSS Users Group, February 1980.  Available from
      the DCPU, National Physical Laboratory, Teddington, UK.

August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           

   [7]  X.25

      CCITT, "Recommendation X.25 - Interface Between Data Terminal
      Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
      Terminals Operating in the Packet Mode on Public Data Networks,"
      CCITT Orange Book, Vol. VIII.2, International Telephone and
      Telegraph Consultative Committee, Geneva, 1976.


RFC821  SIMPLE MAIL TRANSFER PROTOCOL                          RFC821 簡單郵件傳輸協議（SMTP）

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