[轉貼] SMTP 指令說明
http://www.cqinc.com.tw/coopermaa/932-DC/practice/SMTPCmds.htm<br><p class="Title" align="center">SMTP 指令說明</p>
<p class="SubTitle2">Summary</p>
<p class="text11">儘管 WWW 和 IM (Instant Messaging) 相繼起而流行，E-Mail
這個最為古老、行之有年的網路服務，還是公認為目前最有效的溝通工具。為了要發出一封信件，Mail Client 會與遠端的 Mail Server
建立起通信的管道，它們彼此之間你來我往資料傳送時，共同遵守著同一套規則： SMTP 協定 (Simple Mail Transfer Protocol)。</p>
<p class="text11">底下說明幾個常用的 SMTP 指令，還有 Mail Server 回傳代碼的意義。</p>
<blockquote>
<p class="text11"><font color="#626262">註：其實可以利用 telnet 工具，讓 telnet 程式扮演
Mail Client 的角色來與 Mail Server 建立連線、溝通並發出信件，方便練習這邊所介紹的 SMTP 指令。可參考這份<a href="http://www.cqinc.com.tw/coopermaa/932-DC/practice/winmail.rar">影片</a>。</font></p>
</blockquote>
<p class="SubTitle2">SMTP Commands</p>
<p class="text11">下列是 Mail Client 用來與 Server 溝通的指令。這些指令沒有大小寫的區別(not case
sensitive)。</p>
<table id="table1" width="90%" border="0">
<tbody><tr>
<td class="etext9" width="80" nowrap="nowrap">
<p class="etext9"><b>HELO</b></p></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">
<p class="text11">啟始與 Mail Server 之間的對話。一般來說，HELO 是 Mail client
送出的第一個指令，用途是告訴 Mail Server 來者為何，通常我們會把電腦的 domain name 告訴 Server。例如: </p>
<blockquote>
<p class="etext9"><b>Helo mail.companyName.com</b></p>
</blockquote>
</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>MAIL FROM</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">告訴 Server 發信人是誰，例如：<blockquote>
<p class="etext9"><b>MAIL FROM: google@yahoo.com</b></p>
</blockquote>
<p>
注意，這邊的發信人郵件地址並不會顯示在信件的寄件者欄中，當信件無法順利送到目的地時，退回的信件就會寄到這個地方。（寄件者欄、收件者欄、標題欄、副本、密件副本欄，這些是在
Message 的 Header 中指定的。有關 Mail Message 的格式，請參考
<a href="http://www.cqinc.com.tw/coopermaa/932-DC/practice/0427-EMailService.ppt">Email Service</a> 投影片。)</p></td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>RCPT TO</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">告訴 Server 收件人是誰，例如：<blockquote>
<p class="etext9"><b>RCPT TO: yahoo@google.com</b></p>
</blockquote>
<p>如果需要同時寄一封信給很多人，多下幾次 RCPT TO 指令即可。如果給的不是完整的 e-mail address，代表要寄信到該台
Mail Server 的指定信箱，例如：</p>
<blockquote>
<p class="etext9"><b>RCPT TO: kimo</b></p>
</blockquote>
<p>意思就是要寄信到連線中這台 Mail Server 的 kimo 這個信箱。有一點要注意的是，為了避免成為發垃圾信的工具，大部份 Mail
Server 是不允許你透過它寄信到其它地方的，除非先經過<a href="http://www.cqinc.com.tw/coopermaa/932-DC/practice/SMTPCmds.htm#SMTP_Authentication">身份的確認</a>。</p></td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>DATA</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">告訴 Server 你已經準備好要開始輸入信件的本文了(Mail Message, 包括 Message
Header 與 Message Body)。要結束 Message 的輸入時，在信件結尾輸入個 . 點符號即可。</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>NOOP</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">NOOP 是『NO OPeration 』的縮寫，用途是請 Server 回應一下，藉此讓 Client
得以確定與 Server 的連線還在。</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>QUIT</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">結束對話，關閉與 Server 間的連線。</td>
</tr>
</tbody></table>
<p class="SubTitle2">SMTP Server Reply codes</p>
<p class="text11">每當 Mail Client 丟出一個指令，Server 這端就會回應一個固定格式的訊息：一個 3
位數的數字，然後緊接的一串文字描述，說明 Server 收到 client 端傳來的指令之後，Server 處理完後的情形，例如：</p>
<blockquote>
<p class="etext9"><b>250 ok</b></p>
</blockquote>
<p class="text11">或是</p>
<blockquote>
<p class="etext9"><b>221 Bye bye</b></p>
</blockquote>
<p class="text11" align="left">底下列出 Mail Server 回傳的各個代碼以及文字說明(只要下的 SMTP
指令正確，底下絕大部份的代碼應該都不會遇到的！)：</p>
<table id="table2" width="90%" border="0" cellpadding="0" cellspacing="0">
<tbody><tr>
<td class="etext9" width="80" nowrap="nowrap">
<p class="etext9"><b>221</b></p></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">A system status or help reply</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>214</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">Help Message</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>220</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The server is ready. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>221</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The server is ending the conversation. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>250</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The requested action was completed</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>251</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The specified user is not local, but the server will
forward the mail message</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>354</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">This is a reply to the DATA command. After getting
this, start sending the body of the mail message, ending with
"\r\n.\r\n." </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>421</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The mail server will be shut down. Save the mail
message and try again later</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>450</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The mailbox that you are trying to reach is busy.
Wait a little while and try again. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>451</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The requested action was not done. Some error
occurmiles in the mail server</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>452</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The requested action was not done. The mail server
ran out of system storage. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>500</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The last command contained a syntax error or the
command line was too long. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>501</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The parameters or arguments in the last command
contained a syntax error. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>502</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The mail server has not implemented the last command.
</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>503</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The last command was sent out of sequence. For
example, you might have sent DATA before sending RECV</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>504</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">One of the parameters of the last command has not
been implemented by the server. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>550</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The mailbox that you are trying to reach can't be
found or you don't have access rights. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>551</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The specified user is not local; part of the text of
the message will contain a forwarding address. </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>552</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The mailbox that you are trying to reach has run out
of space. Store the message and try again tomorrow or in a few
days-after the user gets a chance to delete some messages.</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>553</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The mail address that you specified was not
syntactically correct</td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"><b>554</b></td>
<td class="text11"> </td>
</tr>
<tr>
<td class="etext9" width="80" nowrap="nowrap"> </td>
<td class="text11">The mail transaction has failed for unknown causes.
</td>
</tr>
</tbody></table>
<p class="SubTitle2"><a name="SMTP_Authentication">SMTP Authentication</a></p>
<p class="text11">由於 SMTP 協定極為簡單，任何人只要懂得 SMTP 指定，就有可能寫出自動發廣告信、垃圾信的程式。為了防堵這些日益氾濫的垃圾信件，後來就出現了加強版的 ESMTP 協定 (Enhanced SMTP )，支援 ESMTP 的 Mail Server
會對那些和它連線的 Mail Client 有較為嚴格的要求，在某些狀況下，例如要透過 Server
發信到其它地方，而非寄到本地的信箱時，限制他們必須先通過身份的驗證，才允許發信出去。（換句話說，如果你透過了身份的確認，要發信到什麼地方都是可以的！）</p>
<p class="text11">一般而言，現在網路上的 Mail Server 主要支援兩種驗證身份的方式： 1. 明碼驗證 和 2. base64
編碼驗證。</p>
<ol><li>
<p class="text11">明碼驗證</p></li></ol>
<blockquote>
<p class="text11">明碼驗證指的是，當 Client 要跟 Server 進行身份驗證時，傳送的 username 和 password 不需要經過任何編碼，只要送給
Server 端 AUTH PLAIN 這個指令後，再直接將 username 和 password 傳給 Server 端，Server
就會進行身份的驗證。(註：大部份 Mail Server 甚至連明碼驗證都覺得不夠安全，因而捨棄這種方式只支援 base64 編碼驗證！）</p>
</blockquote>
<ol><li value="2">
<p class="text11" align="left">base64 編碼驗證</p></li></ol>
<p class="text11">當 Client 端送出 AUTH LOGIN 這個指令到 Server 端時，代表著要使用 base64
編碼的身份驗證。此時 Client 端就必須先將 username 和 password 以 base64 演算法編碼之後，再傳送給 Server
端，Server 這端在拿到編碼字串後先做解碼然後再進行驗證。底下是個 base64 編碼驗證成功的範例畫面，畫面上的 username 和 password 已經使用 base64 演算法編碼過。：</p>
<p class="text11" align="center">
<img src="http://www.cqinc.com.tw/coopermaa/932-DC/practice/images/SMTPCm3.gif" border="0"></p>
<p class="text11">你可以不用自己撰寫 base64 演算法，有一些網站提供了線上
base64 編碼的服務，例如：<a target="_blank" href="http://www.motobit.com/util/base64-decoder-encoder.asp">http://www.motobit.com/util/base64-decoder-encoder.asp</a>。在
ActiveTcl 中，也有 base64 的套件可供使用，底下是個 base64 編碼的範例： </p>
<!-- BEGIN: CUT HERE FOR EMBEDDING -->
<span style="font-family: 'Courier New'; font-size: 10pt; font-weight: 400; color: rgb(64, 64, 64);">
<span style="color: rgb(0, 128, 0);"># Load base64 package</span><br>
<span style="color: rgb(189, 15, 146);">package</span> require base64<br><br>
<span style="color: rgb(0, 128, 0);"># encode username "maa", you'll get "</span></span><font color="#ff0000"><span style="font-family: 'Courier New'; font-size: 10pt; font-weight: 400;">bWFh</span></font><span style="font-family: 'Courier New'; font-size: 10pt; font-weight: 400; color: rgb(64, 64, 64);"><span style="color: rgb(0, 128, 0);">"</span><br>base64<span style="color: rgb(189, 15, 146);">:</span><span style="color: rgb(189, 15, 146);">:</span>encode maa<br>
<br><span style="color: rgb(0, 128, 0);"># encode password "12345", you'll get "</span></span><font color="#ff0000"><span style="font-family: 'Courier New'; font-size: 10pt; font-weight: 400;">MTIzNDU=</span></font><span style="font-family: 'Courier New'; font-size: 10pt; font-weight: 400; color: rgb(64, 64, 64);"><span style="color: rgb(0, 128, 0);">"</span><br>base64<span style="color: rgb(189, 15, 146);">:</span><span style="color: rgb(189, 15, 146);">:</span></span><span style="font-family: 'Courier New'; font-size: 10pt; font-weight: 400; color: rgb(64, 64, 64);">encode </span>
<span style="font-family: 'Courier New'; font-size: 10pt; font-weight: 400; color: rgb(64, 64, 64);">12345</span><br>