Thank you for using Aspose.Email.
MessageId is not something generated by Aspose.Email but is always generated by the sending server that sends out the email. However,the MESSAGE-ID field is considered optional and how the MESSAGE-ID field is created is up to the server as per rfc 2822. For example, when I use the Aspose smtp client to send out an email, the MessageId is always there at the receiving end with the message. I hope this clarifies this functionality.