A Message-ID is an email identifier. It is generated by the client sending the mail and added in the Message-ID header
An message id is generally the concatenation of:
Examples of two differents message id header
Generated by postfix
Message-ID: <[email protected]>
This id seems to follow the algorithm describe in Recommendations for generating Message IDs where:
Generated by Javax Mail
Message-ID: <[email protected]>
Message-ID: <hashcode>.<id>.<currentTime>@<hostname>
where:
With the Gmail original message