MIME邮件格式的浅析

最新推荐文章于 2023-11-15 17:53:02 发布

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#email #application #image #html #parallel #internet

本文介绍了MIME邮件格式的基础知识，包括其发展背景、关键组成部分如邮件头部与主体的编码方式，以及常用的Content-Type类型。重点讲解了multipart/alternative、multipart/mixed和multipart/related的用法。

在RFC 2822文档中定义了简单的 ASCII编码的Email的邮件格式，然而随着Internet的发展，Email邮件仅仅传输简单的文本已经满足不了用户的需求，为了在Email 中传输大量HTML、图像、声音以及各种附件格式，一种新的扩展的邮件格式应运而生——MIME。由于在MIME邮件格式非常复杂，大量的RFC文档中对MIME邮件格式进行了定义与说明，比如RFC2045, RFC2046, RFC2047, RFC2049, RFC2231, RFC2387, RFC4288, RFC4289等。因此，MIME邮件格式为我们提供了极大的灵活性的同时也给我们解读MIME格式的Email邮件带来了极大的困难。以下就一封具体的邮件原始信息对MIME邮件格式作出一个概要的介绍。

解释几个常用且不易理解的Content-Type开头的MIME首部：

Received: from JSJ104 (unknown [202.204.96.147])

by smtp14 (Coremail) with SMTP id wKjRDyJAHALkc2lFEtAjAg==.38990S2;

Sun, 26 Nov 2006 19:00:55 +0800 (CST)

Message-ID: <000601c7114a$2fd50450$9360ccca@JSJ104>

From: <ctp_41023@163.com>

To: <ctp_41023@163.com>

Subject: =?gb2312?B?MTIzMTIzxOO6wzEyMzEyMw==?=

Date: Sun, 26 Nov 2006 19:01:04 +0800

MIME-Version: 1.0

Content-Type: multipart/alternative;

boundary="----=_NextPart_000_0003_01C7118D.38E01920"

X-Priority: 3

X-MSMail-Priority: Normal

X-Mailer: Microsoft Outlook Express 6.00.2900.2869

X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2962

This is a multi-part message in MIME format.

------=_NextPart_000_0003_01C7118D.38E01920

Content-Type: text/plain;

charset="gb2312"

Content-Transfer-Encoding: base64

MTIzMTIzMTIz

------=_NextPart_000_0003_01C7118D.38E01920

Content-Type: text/html;

charset="gb2312"

Content-Transfer-Encoding: base64

PCFET0NUWVBFIEhUTUwgUFVCTElDICItLy9XM0MvL0RURCBIVE1MIDQuMCBUcmFuc2l0aW9uYWwv

L0VOIj4NCjxIVE1MPjxIRUFEPg0KPE1FVEEgaHR0cC1lcXVpdj1Db250ZW50LVR5cGUgY29udGVu

dD0idGV4dC9odG1sOyBjaGFyc2V0PWdiMjMxMiI+DQo8TUVUQSBjb250ZW50PSJNU0hUTUwgNi4w

MC4yOTAwLjI5OTUiIG5hbWU9R0VORVJBVE9SPg0KPFNUWUxFPjwvU1RZTEU+DQo8L0hFQUQ+DQo8

Qk9EWSBiZ0NvbG9yPSNmZmZmZmY+DQo8RElWPjxGT05UIHNpemU9Mj4xMjMxMjMxMjM8L0ZPTlQ+

PC9ESVY+PC9CT0RZPjwvSFRNTD4NCg==

------=_NextPart_000_0003_01C7118D.38E01920--

这封Email具有一个邮件首部，然后是邮件的主体部分，它包括一个文本和一个HTML类型。可以注意到这样一行编码“boundary="----=_NextPart_000_0003_01C7118D.38E01920" ”，它是用来隔离MIME邮件的各个实体部分。

在RFC2822中定义了MIME邮件首部的格式：

field-name ":" [ field-body ] CRLF

例如：
MIME-Version: 1.0
Content-Type: multipart/alternative;
Content-Type: text/plain;
Content-Transfer-Encoding: base64

最常见也用途最都的MIME首部是以Content-Type开头的，在RFC2046给出了它的定义，大致与如下内容相似：

Content-Type: text/plain;
Content-Type: text/plain; charset=ISO-8859-1
Content-Type: text/plain; charset=us-ascii
Content-Type: text/plain; charset=utf-8
Content-Type: text/html;
Content-Type: text/html; charset=ISO-8859-1
Content-Type: text/css
Content-Type: image/gif; name=image004.gif
Content-Type: image/jpeg; name="image005.jpg"
Content-Type: message/delivery-status
Content-Type: message/rfc822
Content-Type: audio/x-mpeg
Content-Type: video/mpeg-2
Content-Type: application/msword
Content-Type: application/mspowerpoint
Content-Type: application/zip

Content-Type: multipart/mixed;     boundary="----=_Part_3431_12384933.1139387792352"
Content-Type: multipart/alternative; boundary="----=_Part_4088_29304219.1115463798628"
Content-Type: multipart/related;     boundary="----=_Part_2067_9241611.1139322711488"
Content-Type: multipart/digest;     boundary="----=Next message 15543233913938263541"
Content-Type: multipart/report; report-type=delivery-status;
              boundary="k04G6HJ9025016.1136391237/carbon.singnet.com.sg"
Content-Type: multipart/parallel