Email headers offer access to many an email's detail, such as its path, email program or spam filtering information. In OS X Mail, you do not have to open the full message source to access all header lines for a message.
View All Email Headers in Mac OS X Mail
To have OS X Mail display all an email message's header lines:
- Open the message in the reading pane or its own window.
- Select View | Message | All Headers from the menu.
- You can also press Command-Shift-H (think "h-eaders", of course).
To revert to the message in regular display:
- Select View | Message | All Headers again from the menu or press Command-Shift-H.
(Updated March 2013)