http://hq.postman.i2p?page_id=8
Usually in the human readable form: Day, Date Month Year, Time (OFFSET to UTC ) Message-ID: An unique identifier for the message. Normally it’s up to the MUA to create a message-ID. If the MUA does not create one, the MTA will insert the header line instead. Message-IDs can contain sensitive information, like local host names, local user names, local dates or timezones.