Be sure to include the entire text of the junk email at the end of your message. This serves at least two purposes. It lends creditability to your complaint as you have sent the evidence. A sure way to include the junk email is to write it out to a file. Then, while you are editing your complaint message, just include the junk email file at the bottom using your editor.
Different editors vary with their commands so find out how to do an include file.
Do not attach the junk email message. Attachments tend to get encrypted before they're sent and you cannot assume root or postmaster will have the necessary software to decrypt your attachment. These are very busy people. Make sure your complaint message is all plain text. Keep your complaint message businesslike and courteous.