The Mail app on macOS and OS X can show you all of the header lines of an email, which contain potentially important and usually hidden information. You may never need to look at the header lines, but if you ever have trouble with your email, a technician may ask you for information hidden in the normally hidden header.
How to Show Full Email Headers in Mail for Mac OS X
Email headers provide access to many details about an email, such as the path, email client, and spam filter information. In Mail, you don't have to open the entire message source to access all the header lines for a message.
You can get a view of all the normally hidden header lines right within the message itself. Once there, you can look for X-Unsubscribe information, for example, which will tell you how to unsubscribe from an email list, or look at Received lines to see the path an email took to get from the sender to your Mail inbox.
To have the Mac Mail app display all email headers for a specific email message, do the following:
