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邮件格式 (RFC 822)

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邮件是由纯文本组成,其详细的格式有很多 RFC 规范需要遵守。我这里只能对我所了解的,也是基础的 —— 或者说最核心的 —— 格式做一个说明。

最核心的部分是 1982 年的 RFC 822 (STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES),之后又做过一些更新, 比如 RFC 2822RFC 5322 (Internet Message Format) 和一堆补丁更新。本文要讲的基本格式,从开始到现在并没有什么明显变化。

  1. 邮件是一种纯文本格式,最开始只包含 ASCII 字符,后来引入了 MIME 之后,可以制定别的编码,比如 UTF-8 等。

  2. 换行符是 \r\n,也就是 CR + LF

  3. 整体来说,一封邮件由邮件头(Headers)和邮件体(Payload)组成。

  4. 邮件头包含若干个头字段

  5. 邮件头和邮件体之间用一个空行隔开

  6. RFC2882 和 RFC5322 都规定了电子邮件每一行的长度,排除行末 CRLF,不可以超过 998 个字符,建议不超过 78 个字符。

There are two limits that this specification places on the number of
characters in a line.  Each line of characters MUST be no more than
998 characters, and SHOULD be no more than 78 characters, excluding
the CRLF.

如果太长,应该拆分成多行,下一行行首加上至少一个空格或者制表符,表示是上一行的延续。

邮件示例

From: Bob <bob@markjour.com>
To: Mark <mark@markjour.com>
Subject: Hello

Hello, Bob,

Would you like to join me for dinner?

--
Mark

RFC 822 中关于字符的定义

                                            ; (  Octal, Decimal.)
# 字符
CHAR        =  <any ASCII character>        ; (  0-177,  0.-127.)
# 字母
ALPHA       =  <any ASCII alphabetic character>
                                            ; (101-132, 65.- 90.)
                                            ; (141-172, 97.-122.)
# 数字
DIGIT       =  <any ASCII decimal digit>    ; ( 60- 71, 48.- 57.)
# 控制字符
CTL         =  <any ASCII control           ; (  0- 37,  0.- 31.)
                character and DEL>          ; (    177,     127.)
# 回车
CR          =  <ASCII CR, carriage return>  ; (     15,      13.)
# 换行
LF          =  <ASCII LF, linefeed>         ; (     12,      10.)
# 空格
SPACE       =  <ASCII SP, space>            ; (     40,      32.)
# 制表符
HTAB        =  <ASCII HT, horizontal-tab>   ; (     11,       9.)
# 引号
<">         =  <ASCII quote mark>           ; (     42,      34.)
# 回车换行
CRLF        =  CR LF
# 空白
LWSP-char   =  SPACE / HTAB                 ; semantics = SPACE
# 连贯空白, 折行空白
linear-white-space =  1*([CRLF] LWSP-char)  ; semantics = SPACE
                                            ; CRLF => folding
# 特殊字符
specials    =  "(" / ")" / "<" / ">" / "@"  ; Must be in quoted-
            /  "," / ";" / ":" / "\" / <">  ;  string, to use
            /  "." / "[" / "]"              ;  within a word.
# 分隔符
delimiters  =  specials / linear-white-space / comment
# 文本
text        =  <any CHAR, including bare    ; => atoms, specials,
                CR & bare LF, but NOT       ;  comments and
                including CRLF>             ;  quoted-strings are
                                            ;  NOT recognized.
# 原子字符
atom        =  1*<any CHAR except specials, SPACE and CTLs>
quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or
                                            ;   quoted chars.
qtext       =  <any CHAR excepting <">,     ; => may be folded
                "\" & CR, and including
                linear-white-space>
domain-literal =  "[" *(dtext / quoted-pair) "]"
dtext       =  <any CHAR excluding "[",     ; => may be folded
                "]", "\" & CR, & including
                linear-white-space>
# 注释
comment     =  "(" *(ctext / quoted-pair / comment) ")"
ctext       =  <any CHAR excluding "(",     ; => may be folded
                ")", "\" & CR, & including
                linear-white-space>
quoted-pair =  "\" CHAR                     ; may quote any char
phrase      =  1*word                       ; Sequence of words
word        =  atom / quoted-string

对应上 ASCII:

  0 -  31 控制字符, 其中包括常用的:
          - HT ( 9)  水平制表符
          - LF (10) 换行
          - CR (13) 回车
       32 空格
 33 -  47 符号 !"#$%&'()*+,-./
 48 -  57 数字
 58 -  64 符号 :;<=>?@
 65 -  90 大写字母
 91 -  96 符号 [\]^_`
 97 - 122 小写字母
123 - 126 符号 {|}~
      127 控制字符(DEL)

CHAR 0-127
CTL  0-37 + 127

符号中:
()<>[]@,;:\".
    13 个被视作特殊字符,需要转义
!#$%&'*+-/=?^_`
    15 个就是普通符号

atom = 数字 + 字母 + 普通符号

邮件头格式

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>

字段名称允许使用的字符范围非常宽泛,而且大小写不敏感,

但是一般实践中:

  • 由字母和中划线组成
  • 首字母大写,或全部大写

常用字段

  • Received MTA 轨迹(传输过程中的相关信息)
  • Date 发信时间,格式:Fri, 21 Nov 1997 09:55:06 -0600
  • Sender Mail From 地址
  • From 发件人
  • Subject 邮件标题
  • To 收件人
  • Cc 抄送
  • Bcc 密送
  • Reply-To 回复地址
  • Message-ID 邮件标识
  • References 回复邮件标识,逗号隔开
  • In-Reply-To 回复邮件标识(会话发起的第一封)
  • Return-Path 发信任地址(2020/07/31,邮件的 Return-Path 头是什么
  • Comments 说明
  • Keywords 关键字

一般采用 X- 开头的字段名称表示自定义字段,或者叫拓展字段:

常见的拓展字段:

  • X-Mailer
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