Think you may have a reverse license check when creating a msg file with attachment(s). When providing a valid license file, the attachments are simply not included in the created mail.
Very easily tested with the provided code from the tutorials section (Spire.Email-Program-Guide/Creating-MSG-Files-with-RTF-body-and-attachment.html). If you run the same code with a valid license file, the email is created without attachment. Please advice.