Message-ID is a unique identifier for a digital message, most commonly a globally unique identifier used in email and Usenet newsgroups.
[1] Message-IDs are required to have a specific format which is a subset of an email address[2] and be globally unique.
Such a problem has been reported with Google MTAs mangling Message-IDs created by Outlook, making it difficult to reference other messages and breaking threading.
[citation needed] Message-IDs, if present, are generated by the client program sending the email[2] or by the first mail server.
[3] A common method of generating such ID is by combining the time and domain name, for example: 950124.162336@example.com.