An email message internet header provides a list of technical details about the message, such as who sent it, the software used to compose it, and the email servers that it passed through on its way to the recipient. Most of the time, only an administrator will need to view internet headers for a message.
View internet message headers
In Outlook for Office 365, 2016, 2013, or 2010 on a PC
Double-click an email message to open it outside of the Reading Pane.
Click File > Properties.
Header information appears in the Internet headers box.
Tip: You can highlight the information in that box, press Ctrl+C to copy, and paste it into Notepad or Word to see the entire header at once.