[PHP-users 30420] Message-ID (Re: サイトからメールの送信が出来ない場合があります )

[DeepOne]

本筋とは特に関係はありませんが、RFC2822によれば

The message identifier (msg-id) itself MUST be a globally
unique identifier for a message. The generator of the
message identifier MUST guarantee that the msg-id is
unique. There are several algorithms that can be used to
accomplish this. Since the msg-id has a similar syntax to
angle-addr (identical except that comments and folding
white space are not allowed), a good method is to put the
domain name (or a domain literal IP address) of the host
on which the message identifier was created on the right
hand side of the "@", and put a combination of the current
absolute date and time along with some other currently
unique (perhaps sequential) identifier available on the
system (for example, a process id number) on the left hand
side. Using a date on the left hand side and a domain name
or domain literal on the right hand side makes it possible
to guarantee uniqueness since no two hosts use the same
domain name or IP address at the same time. Though other
algorithms will work, it is RECOMMENDED that the right
hand side contain some domain identifier (either of the
host itself or otherwise) such that the generator of the
message identifier can guarantee the uniqueness of the
left hand side within the scope of that domain.
(http://www.apps.ietf.org/rfc/rfc2822.html)

となっています。
つまり、Message-IDが世界的に唯一であることを保証できるな
ら@の左右をどのように書くかは明確には規定されていません
。（右側にドメインを含むことは推奨されています。）

定義的にも
msg-id          =       [CFWS] "<" id-left "@" id-right
">" [CFWS]
id-left         =       dot-atom-text / no-fold-quote /
obs-id-left
id-right        =       dot-atom-text / no-fold-literal /
obs-id-right
ですから、規制は非常に緩いものです。

ちなみに、obs-id-rightはdomainと規定されています。

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