Gets the unique message identifier assigned by the mail server.
A string containing the unique message identifier assigned by the mail server, or an empty string if id field is missing in the Received header.
This value is neither MessageID nor Unique-ID assigned by the destination mail server to the message when it gets added into the mailbox. Also, if the message contains multiple Received headers, their ID values are usually different.
The example is available in TimeStamp topic.
TimeStamp Class | MailBee.Mime Namespace