rfc1036 December 1987

2.1.5.  Message-ID

    The "Message-ID" line gives the message a unique identifier.  The
    Message-ID may not be reused during the lifetime of any previous
    message with the same Message-ID.  (It is recommended that no
    Message-ID be reused for at least two years.)  Message-ID's have the
    syntax:

           <string not containing blank or ">">

    In order to conform to RFC-822, the Message-ID must have the format:

                <unique@full_domain_name>

    where full_domain_name is the full name of the host at which the
    message entered the network, including a domain that host is in, and
    unique is any string of printing ASCII characters, not including "<"
    (left angle bracket), ">" (right angle bracket), or "@" (at sign).
    For example, the unique part could be an integer representing a
    sequence number for messages submitted to the network, or a short
    string derived from the date and time the message was created.  For
    example, a valid Message-ID for a message submitted from host ucbvax
    in domain "Berkeley.EDU" would be "<4123@ucbvax.Berkeley.EDU>".
    Programmers are urged not to make assumptions about the content of
    Message-ID fields from other hosts, but to treat them as unknown
    character strings.  It is not safe, for example, to assume that a
    Message-ID will be under 14 characters, that it is unique in the
    first 14 characters, nor that is does not contain a "/".

    The angle brackets are considered part of the Message-ID.  Thus, in
    references to the Message-ID, such as the ihave/sendme and cancel
    control messages, the angle brackets are included.  White space
    characters (e.g., blank and tab) are not allowed in a Message-ID.
    Slashes ("/") are strongly discouraged.  All characters between the
    angle brackets must be printing ASCII characters.