内容转换Content conversion

“内容转换”是针对每个收件人正确设置邮件格式的过程。Content conversion is the process of correctly formatting a message for each recipient. 对邮件执行内容转换的决定取决于邮件的目标地址和格式。The decision to perform content conversion on a message depends on the destination and format of the message. 2016 和 Exchange 2019 Exchange 2019 中发生的内容转换类型与 2013 Exchange无变化:They types of content conversion that occur in Exchange 2016 and Exchange 2019 are unchanged from Exchange 2013:

  • 外部收件人 的邮件转换:此类型的内容转换包括传输中性封装格式 (TNEF) 外部收件人的邮件编码选项。Message conversion for external recipients: This type of content conversion includes the Transport Neutral Encapsulation Format (TNEF) conversion options and message encoding options for external recipients. 发送到 Exchange 组织内部的收件人的邮件不要求此类型的内容转换。Messages sent to recipients inside the Exchange organization don't require this type of content conversion. 此类型的内容转换由邮箱服务器上传输服务中的分类程序处理。This type of content conversion is handled by the categorizer in the Transport service on a Mailbox server. 在将新到达的邮件放入提交队列之后,对每封邮件进行分类。Categorization on each message happens after a newly arrived message is put in the Submission queue. 除了收件人解析和路由解析外,还要对邮件执行内容转换,然后再将该邮件放入传递队列。In addition to recipient resolution and routing resolution, content conversion is performed on the message before the message is put in a delivery queue. 如果单个邮件包含多个收件人,则分类程序会针对每个邮件收件人确定相应的编码。If a single message contains multiple recipients, the categorizer determines the appropriate encoding for each message recipient. 但是,内容转换跟踪不会捕获分类程序转换发送到外部收件人的邮件时遇到的任何内容转换失败。Content conversion tracing doesn't capture any content conversion failures that the categorizer encounters as it converts messages sent to external recipients.

  • 内部收件人的 MAPI 转换:此类型的内容转换由邮箱传输服务处理。MAPI conversion for internal recipients: This type of content conversion is handled by the Mailbox Transport service. 邮箱传输服务位于邮箱服务器上,用于在本地服务器上邮箱数据库和邮箱服务器上的传输服务之间传输邮件。The Mailbox Transport service exists on Mailbox servers to transmit messages between mailbox databases on the local server, and the Transport service on Mailbox servers. 具体来说,邮箱传输提交服务将邮件从发件人的发件箱传输到邮箱服务器上的传输服务。Specifically, the Mailbox Transport Submission service transmits messages from the sender's Outbox to the Transport service on a Mailbox server. 邮箱传输传递服务将邮件从邮箱服务器的传输服务传输到收件人的收件箱。The Mailbox Transport Delivery service transmits messages from the Transport service on a Mailbox server to the recipient's Inbox. 邮箱传输提交服务转换 MAPI 的所有传出邮件,邮箱传输传递服务将所有传入邮件转换为 MAPI。The Mailbox Transport Submission service converts all outgoing messages from MAPI and the Mailbox Transport Delivery service converts all incoming messages to MAPI. 内容转换跟踪捕获这些 MAPI 转换失败。Content conversion tracing captures these MAPI conversion failures. 有关详细信息,请参阅Managing Content Conversion TracingFor more information, see Managing Content Conversion Tracing.

Exchange 和 Outlook 邮件格式Exchange and Outlook message formats

以下列表介绍了在邮件和邮件Exchange Outlook:The following list describes the basic message formats available in Exchange and Outlook:

  • 纯文本:纯文本邮件仅使用 RFC 5322 中所述的 US-ASCII 文本。Plain text: A plain text message uses only US-ASCII text as described in RFC 5322. 这种邮件不能包含其他的字体或其他文本格式。The message can't contain different fonts or other text formatting. 纯文本邮件可以使用以下两种格式:The following two formats can be used for a plain text message:

    • 邮件头和邮件正文由 US-ASCII 文本组成。必须使用 Uuencode 对附件进行编码。Uuencode 指的是“Unix 到 Unix”的编码,它定义使用 US-ASCII 文本字符存储电子邮件正文中的二进制附件的编码算法。The message headers and the message body are composed of US-ASCII text. Attachments must be encoded by using Uuencode. Uuencode represents Unix-to-Unix encoding and defines an encoding algorithm to store binary attachments in the body of an email message by using US-ASCII text characters.

    • 邮件使用 Content-Type 值的 MIME 编码,以及多部分邮件的文本部分的 text/plain Content-Transfer-Encoding 7bit 值。The message is MIME-encoded with a Content-Type value of text/plain, and a Content-Transfer-Encoding value of 7bit for the text parts of a multipart message. 任何邮件附件都使用 Quoted-printable 或 Base64 编码进行编码。Any message attachments are encoded by using Quoted-printable or Base64 encoding. 默认情况下,当您在邮件中撰写和发送纯文本Outlook,邮件会使用 Content-Type 值的 MIME 编码 text/plainBy default, when you compose and send a plain text message in Outlook, the message is MIME-encoded with a Content-Type value of text/plain.

  • HTML:HTML 邮件支持文本格式、背景图像、表、项目符号点和其他图形元素。HTML: An HTML message supports text formatting, background images, tables, bullet points, and other graphical elements. 根据定义,必须对 HTML 格式的邮件进行 MIME 编码以保留这些格式元素。By definition, an HTML-formatted message must be MIME-encoded to preserve these formatting elements.

  • RTF 格式 (RTF) :RTF 支持文本格式和其他图形元素。Rich text format (RTF): RTF supports text formatting and other graphical elements. RTF 与 TNEF 同义 (TNEF,RTF 可与 TNEF 互换) 。RTF is synonymous with TNEF (TNEF and RTF can be used interchangeably). 格式文本消息格式与 Word 中提供的富文本文档格式完全不同。The rich text message format is completely different from the rich text document format that's available in Word.

  • TNEF: 传输中性封装格式是 Microsoft 特定的格式,用于封装 MAPI 邮件属性。TNEF: The Transport Neutral Encapsulation Format is a Microsoft-specific format for encapsulating MAPI message properties. TNEF 邮件包含纯文本形式的邮件以及打包原始格式形式的邮件的附件。A TNEF message contains a plain text version of the message and an attachment that packages the original formatted version of the message. 通常,此附件名为 Winmail.dat。Typically, this attachment is named Winmail.dat. Winmail.dat 附件包含以下信息:The Winmail.dat attachment includes the following information:

    • 邮件的原始格式版本 (例如字体、文本大小和文本颜色) Original formatted version of the message (for example, fonts, text sizes, and text colors)
    • OLE 对象 (,例如嵌入图片或嵌入Office文档) OLE objects (for example, embedded pictures or embedded Office documents)
    • 特殊的Outlook自定义 (,例如自定义窗体、投票按钮或会议) Special Outlook features (for example, custom forms, voting buttons, or meeting requests)
    • 原始邮件中的常规邮件附件Regular message attachments that were in the original message

    最终生成的纯文本邮件可以通过下列格式表示:The resulting plain text message can be represented in the following formats:

    • 符合 RFC 5322 的邮件,仅由 Uuencode 编码的 Winmail.dat 附件的 US-ASCII 文本组成RFC 5322-compliant message composed of only US-ASCII text with a Winmail.dat attachment encoded in Uuencode
    • 带有 Winmail.dat 附件的多节 MIME 编码的邮件Multipart MIME-encoded message that has a Winmail.dat attachment

    Outlook完全理解 TNEF 的电子邮件客户端处理 Winmail.dat 附件并显示原始邮件内容,而不会显示 Winmail.dat 附件。Outlook and other email clients that fully understand TNEF process the Winmail.dat attachment and display the original message content without ever displaying the Winmail.dat attachment. 无法理解 TNEF 的电子邮件客户端可能通过以下任一方式显示 TNEF 邮件:Email clients that don't understand TNEF may present TNEF messages in any of the following ways:

    • 将显示纯文本版本的邮件,并且邮件包含名为 Winmail.dat、Win.dat 或一些其他通用名称(如 Att nnnnn.dat 或 Att nnnnn.eml)的附件,其中 nnnnn 占位符表示随机数。The plain text version of the message is displayed, and the message contains an attachment named Winmail.dat, Win.dat, or some other generic name such as Att nnnnn.dat or Att nnnnn.eml where the nnnnn placeholder represents a random number.
    • 显示纯文本版本的邮件。忽略或删除 TNEF 附件。结果是纯文本邮件。 The plain text version of the message is displayed. The TNEF attachment is ignored or removed. The result is a plain text message.
    • 支持 TNEF 的邮件服务器可以配置为从传入邮件中删除 TNEF 附件。Messaging servers that understand TNEF can be configured to remove TNEF attachments from incoming messages. 结果是纯文本邮件。The result is a plain text message. 此外,某些电子邮件客户端可能无法理解 TNEF,但可识别并忽略 TNEF 附件。Moreover, some email clients may not understand TNEF, but recognize and ignore TNEF attachments. 结果是纯文本邮件。The result is a plain text message.

    有一些第三方实用程序可以帮助转换 Winmail.dat 附件。There are third-party utilities that can help convert Winmail.dat attachments.

    从 Exchange Server 版本 5.5 开始,所有 Exchange 版本均支持 TNEF。TNEF is understood by all versions of Exchange since Exchange Server version 5.5.

  • STNEF ( 的摘要传输中性封装) :STNEF 等效于 TNEF。Summary Transport Neutral Encapsulation Format (STNEF): STNEF is equivalent to TNEF. 但是,STNEF 邮件的编码方式与 TNEF 邮件不同。However, STNEF messages are encoded differently than TNEF messages. 具体来说,STNEF 邮件始终采用 MIME 编码,并且始终具有 Content-Transfer-EncodingBinarySpecifically, STNEF messages are always MIME-encoded, and always have the Content-Transfer-Encoding value Binary. 所以,该邮件没有纯文本表示形式,并且该邮件的正文中没有确切的 Winmail.dat 附件。Therefore, there's no plain text representation of the message, and there's no distinct Winmail.dat attachment contained in the body of the message. 整个邮件只使用二进制数据表示。The whole message is represented by using only binary data. Content-Transfer-Encoding 值为 Binary 的邮件只能在支持并公布 BINARYMIMECHUNKING SMTP 扩展的邮件服务器之间传输,如 RFC 3030 中的定义。Messages that have a Content-Transfer-Encoding value of Binary can only be transferred between messaging servers that support and advertise the BINARYMIME and CHUNKING SMTP extensions as defined in RFC 3030. 消息始终使用 BDAT 命令(而不是标准 DATA 命令)在邮件服务器 之间 传输。The messages are always transferred between messaging servers by using the BDAT command, instead of the standard DATA command.

    从 Exchange 2000 开始,所有 Exchange 版本均支持 STNEF。从本机模式 Exchange Server 2003 开始,STNEF 自动用于组织中 Exchange 服务器之间传输的所有邮件。STNEF is understood by all versions of Exchange since Exchange 2000. STNEF is automatically used for all messages transferred between Exchange servers in the organization since native mode Exchange Server 2003.

    Exchange 从不将 STNEF 邮件发送到外部收件人。只有 TNEF 邮件可以发送到 Exchange 组织外的收件人。Exchange never sends STNEF messages to external recipients. Only TNEF messages can be sent to recipients outside the Exchange organization.

外部收件人的内容转换选项Content conversion options for external recipients

您可以在 Exchange 组织中为外部收件人设置的内容转换选项可以分为以下几类:The content conversion options that you can set in an Exchange organization for external recipients can be described in the following categories:

  • TNEF 转换 选项:这些转换选项指定应保留 TNEF 还是从离开组织的邮件Exchange TNEF。TNEF conversion options: These conversion options specify whether TNEF should be preserved or removed from messages that leave the Exchange organization.
  • 邮件编码选项:这些选项指定邮件编码选项,如 MIME 和非 MIME 字符集、邮件编码和附件格式。Message encoding options: These options specify message encoding options, such as MIME and non-MIME character sets, message encoding, and attachment formats.

这些转换和编码选项相互独立。例如,TNEF 邮件是否可以从 Exchange 组织发出与这些邮件的 MIME 编码设置或纯文本编码设置无关。These conversion and encoding options are independent of one another. For example, whether TNEF messages can leave the Exchange organization isn't related to the MIME encoding settings or plain text encoding settings of those messages.

您可以在各种 Exchange 组织级别指定内容转换,如下面列表所示:You can specify the content conversion at various levels of the Exchange organization as described in the following list:

  • 远程域设置:远程域定义在组织与外部域Exchange传出邮件传输的设置。Remote domain settings: Remote domains define the settings for outgoing message transfers between the Exchange organization and external domains.. 即使没有为特定域创建远程域条目,也会存在一个名为 Default 的预定义远程域,它适用于所有远程地址空间 ( * )。Even if you don't create remote domain entries for specific domains, there's a predefined remote domain named Default that applies to all remote address spaces (*). 有关远程域的信息,请参阅远程 For more information about remote domains, see Remote Domains.

  • 邮件用户和邮件联系人设置:邮件用户和邮件联系人相似,因为二者都有外部电子邮件地址,并且都包含有关组织外部Exchange的信息。Mail user and mail contact settings: Mail users and mail contacts are similar because both have external email addresses and contain information about people outside the Exchange organization. 主要区别是邮件用户具有可用于登录到 Active Directory 和访问组织中资源的帐户。The main difference is mail users have accounts that they can use to log on to Active Directory and access resources in the organization. 有关详细信息,请参阅 收件人For more information, see Recipients.

  • Outlook设置:可以在以下位置设置这些邮件Outlook:Outlook settings: You can set these message formatting and encoding options in Outlook:

    • 邮件 格式:可以设置所有邮件的默认邮件格式。Message format: You can set the default message format for all messages. 撰写特定邮件时,您可以覆盖默认邮件格式。You can override the default message format as you compose a specific message.
    • Internet 邮件 格式 :您可以控制是否将 TNEF 邮件发送到远程收件人,或者是否先将其转换为更兼容的格式。Internet message format: You can control whether TNEF messages are sent to remote recipients or whether they are first converted to a more compatible format. 您还可以为发送到远程收件人的邮件指定各种邮件编码选项。You can also specify various message encoding options for messages sent to remote recipients. 这些设置不适用于发送到 Exchange 组织中的邮件。These settings don't apply to messages sent to recipients in the Exchange organization.
    • Internet 收件人邮件格式 (Outlook 2010 或更早版本) :您可以控制是否将 TNEF 邮件发送到"联系人"文件夹中的特定联系人。Internet recipient message format (Outlook 2010 or earlier): You can control whether TNEF messages are sent to specific contacts in your Contacts folder. 这些转换选项不适用于 Exchange 组织中的收件人。These conversion options aren't available for recipients in the Exchange organization.
    • Internet 收件人邮件编码选项 (Outlook 2010 或更早版本) :您可以控制"联系人"文件夹中特定联系人的 MIME 或纯文本编码选项。Internet recipient message encoding options (Outlook 2010 or earlier): You can control the MIME or plain text encoding options for specific contacts in your Contacts folder. 这些转换选项不适用于 Exchange 组织中的收件人。These conversion options aren't available for recipients in the Exchange organization.
    • 国际选项:可以控制邮件中使用的字符集。International options: You can control the character sets used in messages.

    有关这些设置的详细信息,请参阅TNEF 转换选项和邮件Exchange Server。For more information about these settings, see TNEF conversion options and Message encoding options in Exchange Server.

了解电子邮件消息的结构Understanding the structure of email messages

为了更好地了解外部收件人的内容转换选项,需要了解电子邮件的结构。SMTP 邮件基于 7 位 US-ASCII 纯文本来撰写和发送电子邮件。标准 SMTP 邮件由下列元素组成:To better understand the content conversion options for external recipients, you need to understand the structure of email messages. An SMTP message is based on plain 7-bit US-ASCII text to compose and send email messages. A standard SMTP message consists of the following elements:

  • 邮件信封:邮件信封在 RFC 5321 中定义。Message envelope: The message envelope is defined in RFC 5321. 邮件信封包括传输和传递邮件所需的信息。The message envelope contains information required to transmit and deliver the message. 收件人从不会看到邮件信封,因为它是由邮件传输进程生成的,实际上并不是邮件内容的一部分。Recipients never see the message envelope, because it's generated by the message transmission process and isn't actually part of the message contents.

  • 邮件内容:邮件内容在 RFC 5322 中定义。Message contents: The message contents are defined in RFC 5322. 邮件内容由下列元素组成:The message contents consist of the following elements:

    • 邮件头:邮件头是头字段的集合。Message header: The message header is a collection of header fields. 头字段包括字段名称,后跟冒号 (:)、字段正文,最后是回车换行 (CR/LF) 字符组合。Header fields consist of a field name, followed by a colon (:) character, followed by a field body, and ended by a carriage return/line feed (CR/LF) character combination.

      字段名称必须由可打印的 US-ASCII 文本字符(冒号 (:) 除外)组成。具体来说,允许使用值范围在 33 到 57 和 59 到 126 的 ASCII 字符。A field name must be composed of printable US-ASCII text characters except the colon (:) character. Specifically, ASCII characters that have values from 33 through 57 and 59 through 126 are permitted.

      字段正文可由任何 US-ASCII 字符组成,但回车符 (CR) 和换行符 (LF) 除外。A field body may be composed of any US-ASCII characters, except for the carriage return (CR) character and the line feed (LF) character. 但是,在“头折叠”中使用时,字段正文可以包含 CR/LF 字符组合。However, a field body may contain the CR/LF character combination when used in header folding. 头折叠是按 RFC 5322 的第 2.2.3 节所述,将单个标头字段正文分为多行。Header folding is the separation of a single header field body into multiple lines as described in section 2.2.3 of RFC 5322. RFC 5322 的 3 节和 4 节中介绍了其他字段正文语法要求。Other field body syntax requirements are described in sections 3 and 4 of RFC 5322.

    • 邮件正文:邮件正文是出现在邮件头后的 US-ASCII 文本字符行的集合。Message body: The message body is a collection of lines of US-ASCII text characters that appears after the message header. 邮件头和邮件正文由一个以 CR/LF 字符组合结尾的空行分隔。The message header and the message body are separated by a blank line that ends with the CR/LF character combination. 邮件正文是可选的。The message body is optional. 在邮件正文中,任何文本行都不得超过 998 个字符。Any line of text in the message body must be less than 998 characters. CR 和 LF 字符同时显示才能表示一行的结束。The CR and LF characters can only appear together to indicate the end of a line.

当 SMTP 邮件包含非 US-ASCII 纯文本元素时,必须对其进行编码以保留这些元素。When SMTP messages contain elements that aren't plain US-ASCII text, the message must be encoded to preserve those elements. MIME 标准定义了一种对邮件中非文本内容进行编码的方法。The MIME standard defines a method of encoding content in messages that isn't text. MIME 允许存在使用其他字符集的文本、不带文本的附件、多部分邮件正文以及使用其他字符集的头字段。MIME allows for text in other character sets, attachments without text, multipart message bodies, and header fields in other character sets. MIME 在 RFC 2045、RFC 2046、RFC 2047、RFC 4288、RFC 4289 和 RFC 2049 中定义。MIME is defined in RFC 2045, RFC 2046, RFC 2047, RFC 4288, RFC 4289, and RFC 2049. MIME 定义了指定其他邮件属性的头字段集合。MIME defines a collection of header fields that specifies additional message attributes. 以下各节介绍了一些重要的 MIME 头字段。The following sections describe some important MIME header fields.

MIME-Version头字段MIME-Version header field

默认值:1.0Default value: 1.0

此头字段是 MIME 格式的邮件中出现的第一个 MIME 头字段。This header field is the first MIME header field that appears in a MIME-formatted message. 此头字段出现在其他标准 RFC 5322 头字段之后,但在任何其他 MIME 头字段之前。This header field appears after the other standard RFC 5322 header fields, but before any other MIME header fields. MIME 感知电子邮件客户端使用此头字段标识 MIME 编码的邮件。MIME-aware email clients use this header field to identify a MIME-encoded message. 当缺少此头字段时,MIME 感知电子邮件客户端会将该邮件标识为纯文本。When this header field is absent, MIME-aware email clients identify the message as plain text.

Content-Type 标头字段Content-Type header field

默认值:text/plainDefault value: text/plain

此头字段标识 RFC 2046 中描述的邮件内容的媒体类型。This header field identifies the media type of the message content as described in RFC 2046. 媒体类型包括:A media type consists of:

  • 类型A type:

    • 以 开头 x- 的类型不是标准类型。Types that begin with x- aren't standard. Internet 号码分配机构 (IANA) 维护已注册的媒体类型列表。The Internet Assigned Numbers Authority (IANA) maintains a list of registered media types. 有关详细信息,请参阅MIME Media Types。For more information, see MIME Media Types.

    • 多节 媒体类型通过使用由不同媒体类型定义的节允许在同一邮件中使用多个邮件部分。The multipart media type allows for multiple message parts in the same message by using sections defined by different media types. 些 Content-Type 字段值包括 text/plain 、 、 和 text/html multipart/mixed multipart/alternativeSome Content-Type field values include text/plain, text/html, multipart/mixed, and multipart/alternative.

  • 子类型:以 开头的 vnd. 子类型特定于供应商。A subtype: Subtypes that begin with vnd. are vendor-specific.

  • 一个或多个可选参数:例如,定义 charset= MIME 字符编码的参数。One or more optional parameters: For example, a charset= parameter that defines the MIME character encoding.

Content-Transfer-Encoding 标头字段Content-Transfer-Encoding header field

默认值:7bitDefault value: 7bit

此头字段可以说明有关邮件的以下信息:This header field can describe the following information about a message:

  • 用于转换邮件正文中的任何非 US-ASCII 文本或二进制数据的编码算法。The encoding algorithm used to transform any non-US-ASCII text or binary data that exists in the message body.
  • 描述邮件正文当前情况的指示器。An indicator that describes the current condition of the message body.

MIME 邮件中可以有多个 Content-Transfer-Encoding 头字段值。There can be multiple values of the Content-Transfer-Encoding header field in a MIME message. Content-Transfer-Encoding 头字段出现在邮件头中时,它适用于邮件的整个正文。When the Content-Transfer-Encoding header field appears in the message header, it applies to the whole body of the message. Content-Transfer-Encoding 头字段出现在多部分邮件的某一部分时,它仅适用于邮件的该部分。When the Content-Transfer-Encoding header field appears in one of the parts of a multipart message, it applies only to that part of the message.

如果对邮件正文数据应用编码算法,邮件正文数据将转换为 US-ASCII 纯文本。When an encoding algorithm is applied to the message body data, the message body data is transformed into plain US-ASCII text. 此转换允许邮件通过仅支持 US-ASCII 文本中的邮件的较旧邮件服务器。This transformation allows the message to travel through older messaging servers that only support messages in US-ASCII text. 指示 在邮件正文上使用了编码算法的 Content-Transfer-Encoding 头字段值为:The Content-Transfer-Encoding header field values that indicate an encoding algorithm was used on the message body are:

  • Quoted-printable:使用可打印 US-ASCII 字符对邮件正文数据进行编码。Quoted-printable: Uses printable US-ASCII characters to encode the message body data. 如果原始邮件文本大部分都是 US-ASCII 文本,则 quoted-printable 编码可提供便于阅读的紧凑的显示结果。If the original message text is mostly US-ASCII text, Quoted-printable encoding gives somewhat readable and compact results. 除了等号字符 (=) 之外,所有可打印 US-ASCII 文本字符都可不经过编码而呈现。All printable US-ASCII text characters except the equal sign (=) character can be represented without encoding.

  • Base64:主要基于 RFC 4648 中定义的 (PEM) 隐私增强邮件功能。Base64: Based primarily on the privacy-enhanced mail (PEM) standard defined in RFC 4648. Base64 编码使用 64 个字符字母编码算法和 PEM 定义的输出填充字符对邮件正文数据进行编码。Base64 encoding uses the 64-character alphabet encoding algorithm and output padding characters defined by PEM to encode the message body data. 经过 Base64 编码的邮件通常比原始邮件大 33%。A Base64 encoded message is typically 33 percent larger than the original message. 经 Base64 编码而增加的邮件大小是可预测的,对于二进制数据和非 US-ASCII 文本来说它是最佳选择。Base64 encoding creates a predictable increase in message size and is optimal for binary data and non-US-ASCII text.

通常,您不会看到在同一个邮件中使用多个编码算法。Typically, you won't see multiple encoding algorithms used in the same message.

如果尚未对邮件正文使用编码算法, 则 Content-Transfer-Encoding 头字段仅标识邮件正文数据的当前条件。When no encoding algorithm has been used on the message body, the Content-Transfer-Encoding header field merely identifies the current condition of the message body data. Content-Transfer-Encoding 标头字段值指示邮件正文上未使用编码算法:The Content-Transfer-Encoding header field values that indicate that no encoding algorithms were used on the message body are:

  • 7bit:指示邮件正文数据已采用 RFC 5322 格式。7bit: Indicates that the message body data is already in the RFC 5322 format. 具体来说,这表示必须满足下列条件:Specifically, this means that the following conditions must be true:

    • 所有文本行的长度不得超过 998 个字符。All lines of text must be less than 998 characters long.

    • 所有字符必须是字符值为 1 到 127(包括 1 和 127)的 US-ASCII 文本。All characters must be US-ASCII text that have character values from 1 through 127.

    • CR 和 LF 字符一起使用才能表示一个文本行的结束。The CR and LF characters can only be used together to indicate the end of a line of text.

      整个邮件正文可能是 7 位,或者多部分邮件中的部分邮件正文可能是 7 位。The whole message body may be 7-bit, or part of the message body in a multipart message may be 7-bit. 如果多部分邮件包含具有任何二进制数据或非 US-ASCII 文本的其他部分,则必须使用 Quoted-printable 或 Base64 编码算法对邮件的该部分进行编码。If the multipart message contains other parts that have any binary data or non-US-ASCII text, that part of the message must be encoded using the Quoted-printable or Base64 encoding algorithms.

      具有 7 位主体的邮件可以使用标准 DATA 命令在邮件服务器之间传输。Messages that have 7-bit bodies can travel between messaging servers by using the standard DATA command.

  • 8bit:指示邮件正文数据包含非 US-ASCII 字符。8bit: Indicates that the message body data contains non-US-ASCII characters. 具体来说,这表示必须满足下列条件:Specifically, this means that the following conditions must be true:

    • 所有文本行的长度不得超过 998 个字符。All lines of text must be less than 998 characters long.

    • 邮件正文中一个或多个字符的值大于 127。One or more characters in the message body have values larger than 127.

    • CR 和 LF 字符一起使用才能表示一个文本行的结束。The CR and LF characters can only be used together to indicate the end of a line of text.

      整个邮件正文可能是 8 位,或者多部分邮件中的部分邮件正文可能是 8 位。The whole message body may be 8-bit, or part of the message body in a multipart message may be 8-bit. 如果多部分邮件包含具有二进制数据的其他部分,则必须使用 Quoted-printable 或 Base64 编码算法对邮件的该部分进行编码。If the multipart message contains other parts that have binary data, that part of the message must be encoded using the Quoted-printable or Base64 encoding algorithms.

      具有 8 位主体的邮件只能在支持 RFC 6152 中定义的 8BITMIME SMTP 扩展的邮件服务器(如 Exchange 2000 Server 或更高版本)之间传输。Messages that have 8-bit bodies can only travel between messaging servers that support the 8BITMIME SMTP extension as defined in RFC 6152, such as Exchange 2000 Server or later. 具体来说,这表示必须满足下列条件:Specifically, this means that the following conditions must be true:

    • 8BITMIME 关键字必须在服务器的 EHLO 响应中公布。The 8BITMIME keyword must be advertised in the server's EHLO response.

    • 邮件仍然使用 SMTP 标准 DATA 命令传输。Messages are still transferred by using the SMTP standard DATA command. 但是, BODY=8BITMIME 必须将 参数添加到 MAIL FROM 命令 的末尾。However, the BODY=8BITMIME parameter must be added to the end of the MAIL FROM command.

  • Binary:指示邮件正文包含非 US-ASCII 文本或二进制数据。Binary: Indicates that the message body contains non-US-ASCII text or binary data. 具体来说,这表示必须满足下列条件:Specifically, this means that the following conditions are true:

    • 允许使用任何字符序列。Any sequence of characters is allowed.

    • 行没有长度限制。There is no line length limitation.

    • 二进制邮件元素不需要编码。Binary message elements don't require encoding.

      具有二进制主体的邮件只能在支持 RFC 3030 中定义的 BINARYMIME SMTP 扩展的邮件服务器(如 Exchange 2000 Server 或更高版本)之间传输。Messages that have binary bodies can only travel between messaging servers that support the BINARYMIME SMTP extension as defined in RFC 3030, such as Exchange 2000 Server or later. 具体来说,这表示必须满足下列条件:Specifically, this means that the following conditions must be true:

    • BINARYMIME 关键字必须在服务器的 EHLO 响应中公布。The BINARYMIME keyword must be advertised in the server's EHLO response.

    • BINARYMIME SMTP 扩展只能与 CHUNKING SMTP 扩展一同使用。The BINARYMIME SMTP extension can only be used with the CHUNKING SMTP extension. “分块”允许以多个较小块的形式发送大型邮件正文。Chunking enables large message bodies to be sent in multiple, smaller chunks. 分块也在 RFC 3030 中定义。Chunking is also defined in RFC 3030. CHUNKING 关键字还必须在服务器的 EHLO 响应中公布。The CHUNKING keyword must also be advertised in the server's EHLO response.

    • 消息是使用 BDAT 命令 而不是标准 DATA 命令 传输 的。Messages are transferred using the BDAT command instead of the standard DATA command.

    • 当邮件具有邮件正文时,必须将 参数添加到 BODY=BINARYMIME MAIL FROM 命令的末尾。The BODY=BINARYMIME parameter must be added to the end of the MAIL FROM command when the message has a message body.

值 、 和 从不一起存在于同一 7bit 8bit Binary 多部分消息中 (值相互排斥) 。The values 7bit, 8bit, and Binary never exist together in the same multipart message (the values are mutually exclusive). 或 值可能显示在 7 位或 8 位多部分邮件正文 Quoted-printable Base64 中,但永远不会显示在二进制邮件正文中。The Quoted-printable or Base64 values may appear in a 7-bit or 8-bit multipart message body, but never in a binary message body. 如果多部分邮件正文包含由 7 位和 8 位内容组成的不同部分,则整个邮件被归类为 8 位。If a multipart message body contains different parts composed of 7-bit and 8-bit content, the whole message is classified as 8-bit. 如果多部分邮件正文包含由 7 位、8 位和二进制内容组成的不同部分,则整个邮件被归类为二进制。If a multipart message body contains different parts composed of 7-bit, 8-bit, and binary content, the whole message is classified as binary.

Content-Disposition 标头字段Content-Disposition header field

默认值:AttachmentDefault value: Attachment

此头字段指示启用 MIME 的电子邮件客户端应如何显示附件,在 RFC 2183 中有相关说明。This header field instructs a MIME-enabled email client on how it should display an attached file, and is described in RFC 2183. 有效值为:Valid values are:

  • Inline:附件显示在邮件正文中。Inline: The attachment is displayed in the message body.

  • Attachment:附加的文件显示为独立于邮件正文的常规附件。Attachment: The attached file appears as a regular attachment separate from the message body. 其他参数也包含这些值 (例如 、 和 Filename Creation-date Size) 。Other parameters are also with this values (for example, Filename, Creation-date, and Size).