* 歴史

[FIG[

* RFC 724

[PRE[
         <extension-field> ::=   "In-Reply-To" ":" <reference-list>
                               |...
         <reference-list>  ::=   <null>
                               | <reference-item>
                               | <reference-item> "," <reference-list>
         <reference-item>  ::=   <phrase>
                               | <mach-host-phrase>
         <mach-host-phrase>::=   "<" <host-phrase> ">"
]PRE]


** b. In-Reply-To:

The contents of this field  identify  previous  correspondence
which  this  message answers.  If message identifiers are used
in this field, they should be enclosed in angle brackets (<>).


** 例

- In-Reply-to: <some string at SHOST>

]FIG]

[FIG[
* RFC 733

optional-field  =
[PRE[
               ...
            /  "In-Reply-To"":" #(phrase / mach-id)
            /...
]PRE]


** 2.  In-Reply-To

The contents of this field identify previous correspondence which
this  message answers.  Note that if message identifiers are used
in this field, they must use the mach-id specification format.


** 例

- In-Reply-To: <some string at SHOST>


* RFC 822

[PRE[
     optional-field =
                 ...
                 /  "In-Reply-To"       ":"  *(phrase / msg-id)
                 /...
]PRE]


** 4.6.2.  IN-REPLY-TO

The contents of this field identify  previous  correspondence  
which this message answers.  Note that if message identifiers 
are used in this  field,  they  must  use  the  msg-id
specification format.


** 例

- In-Reply-To: <some.string@DBM.Group>, George's message


** C.3.5.  IN-REPLY-TO

The field-body is no longer a comma-separated list, although a
sequence is still permitted.
]FIG]

** RFC 2822

[[RFC2822の3.6.4]]参照。

[PRE[
  obs-in-reply-to =       "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF
]PRE]

* 関連

[2] [CODE(822)@en[[[References:]]]] 欄も参照。

* メモ

[1] なんとなく [CODE(822)[[[Reply-To]]]] 欄との関係を期待したくなるところですが、あんまり関係ないです。 [CODE(822)[In-Reply-To]] は''そのメッセージが返信する、元メッセージ''を示し、 [CODE(822)[Reply-To]] は''そのメッセージを受け取った人が返信する時に使って欲しい宛先''が示されます。