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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>IMAP</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 4 頁（共6 頁）</nobr></font></td>
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<p>IMAP 是更高級的用于接收消息的協議。在 <a href="http://www.imc.org/rfc2060">RFC 2060</a> 中被定義，IMAP 代表因特網消息訪問協議（Internet Message Access Protocol），目前用的是版本 4，也稱 IMAP4。在用到 IMAP 時，郵件服務器必需支持這個協議。不能僅僅把使用 POP 的程序用于 IMAP，并指望它支持 IMAP 所有性能。假設郵件服務器支持 IMAP，基于 JavaMail 的程序可以利用這種情況 — 用戶在服務器上有多個文件夾（folder），并且這些文件夾可以被多個用戶共享。</p>
<p>因為有這一更高級的性能，您也許會認為所有用戶都會使用 IMAP。事實并不是這樣。
要求服務器接收新消息，在用戶請求時發送到用戶手中，<i>還要</i>在每個用戶的多個文件夾中維護消息。這樣雖然能將消息集中備份，但隨著用戶長期的郵件夾越來越大，到磁盤空間耗盡時，每個用戶都會受到損失。使用 POP，就能卸載郵件服務器上保存的消息了。</p>
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