<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> ">"
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 (<>).
optional-field =
... / "In-Reply-To"":" #(phrase / mach-id) /...
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.
optional-field = ... / "In-Reply-To" ":" *(phrase / msg-id) /...
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.
The field-body is no longer a comma-separated list, although a sequence is still permitted.
obs-in-reply-to = "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF
[2] References:
欄も参照。
[1] なんとなく 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。 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、 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。