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我正在调试一个无法解析来自一个电子邮件服务的标头的 MIME 解析器。电子邮件服务包含一个标题，该标题由一个完全位于下一行的字段正文组成，如下所示：

Message-Id:
  <12345.67890.abcdef@example.com>

这合法吗？

RFC-822 为有效标头指定以下语法：

3.2.  HEADER FIELD DEFINITIONS

      These rules show a field meta-syntax, without regard for the
 particular  type  or internal syntax.  Their purpose is to permit
 detection of fields; also, they present to  higher-level  parsers
 an image of each field as fitting on one line.

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

 field-name  =  1*<any CHAR, excluding CTLs, SPACE, and ":">

 field-body  =  field-body-contents
                [CRLF LWSP-char field-body]

 field-body-contents =
               <the ASCII characters making up the field-body, as
                defined in the following sections, and consisting
                of combinations of atom, quoted-string, and
                specials tokens, or else consisting of texts>

空字符串是否满足 field-body-contents 以允许[CRLF LWSP field-body]field-body 的一部分？

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是的，在空白处折叠是完全允许的。通常，仅当行太长时才这样做，但无论如何它完全是可选的；不需要折叠或避免折叠。（当然，如果一行太长而无法满足 RFC5321，则必须将其折叠或以其他方式缩短。如今，RFC2047 比传统的空白折叠更加通用和透明。）

如https://www.rfc-editor.org/rfc/rfc5322#section-2.2.3中所述，折叠线是一条“逻辑线”。（请注意，旧的 822 早已被更新的标准取代；5322 是当前的替代品。）

于 2015-09-15T07:38:59.367 回答