I start with an unsent Outlook message with a blank Subject. If I open its msg file inside a MailMessage, the msg.Headers collection contains just one item, "X-Unsent" which is an integer value set to 1.
If I save the msg contents to a MemoryStream in DefaultMhtml format (which puts MIME content in the stream) and then use that stream to construct a Aspose.Words.Document object, there is a line that starts with the word Subject: followed by 20 or so spaces, but there is no actual subject text. The other email header items like Sent:, From:, etc. do not appear because they were not present in the original email.
I think it is a bug that Subject: appears in the generated Document object because there is no Subject in the email I started with.
I'd prefer not to have to write special-case code to handle the fact that aspose treats Subject: differently to other email header items.