Lion Mail: Show header details
Each message you receive has a detailed “header” section that shows the route the message took on its way to you, as well as technical details about the message and sender. Your email service provider or network administrator might ask you to send them a message with complete headers when you report problems or junk mail.
If you frequently need to view all headers, add the All Headers icon to the Mail toolbar.
You can create rules that perform actions based on the content of specific headers. For example, if the List-ID header contains “lists.example.com,” you could move the message to a specific folder.
Show header details for a single message
Select the message, and then choose View > Message > All Headers.
Show header details for all messages
Choose Mail > Preferences, click Viewing, and then choose All from the “Show header detail” pop-up menu.
You can customize the fields that Mail displays in header details. Choose Custom from the “Show header detail” pop-up menu, click Add (+), type the field name, and then click OK. To remove a field, select it in the list and click Remove (-).