Missing or malformed Message ID: Since the Message ID includes information about where the message is coming from, it is often missing or malformed (i.e. no @ sign or an empty string) in spam messages. The Message ID is in the form of firstname.lastname@example.org
. The first part can be anything and the second part is the name of the machine that assigned the ID. Although Message ID's are not strictly required, one can safely assume that they would only be missing or malformed if done deliberately to disguise the source of the message.