Viewing Full Headers
The full header of an email message displays information about the sender and the route the message takes before arriving in your inbox.
Creating filters that use the sender's email address is an effective method only if the address is correct. Often spam is sent from fake addresses, so it is ineffective to filter the address listed in the From line. Filters can be more effective if you block a sender's complete address as displayed in the full header.
Also, when reporting spam to a provider, you should always send the full headers along with the message so they can determine where the spam originated.
Procedures for viewing full headers are available for the following email applications: