The method depends on the version of Outlook. Unless you set a registry key, Outlook displays only the headers, not the full message source. If viewing headers is something you need to do often, use a utility or VBA code to view and copy the headers.
Beginning with Outlook 2003, you can view the header and full message source together by adding the SaveAllMIMENotJustHeaders value to the registry. For more information and ready-to-use reg files, see View the Message Source in Outlook
For a more readable copy of the Internet headers in Outlook 2007 and newer, use a VBA macro to create a readable copy the headers in a new message form. Get the code sample from Get Outlook's Internet Headers using VBA.
Outlook 2010 and up
In Outlook 2010 and up, when you use the default settings, you need to OPEN the message and either go to the Tags section of the ribbon or go to File, Properties. Skip past the video to learn how to avoid opening the message to view the header.
Click on the Expand button (highlighted in yellow) in the lower right corner to show the Message options dialog.
View Internet header Video Tutorial
Add Message options to Quick Access Toolbar (QAT)
You can also add the Message options command to the Quick Access Toolbar (QAT) so you don't need to open the message (and can use a Ctrl+n shortcut).
- Go to File > Options > Quick Access Toolbar.
- In Choose Commands From, select Commands Not in the Ribbon
- Click in the list of commands and press M on the keyboard
- Scroll to find Message Options...
- Click Add >> to add it to the QAT.
Add Message options to QAT Video Tutorial
Outlook 2003 and Outlook 2007
Method #1: Right-click the message in the folder view, then choose Options.
Method #2: In an open message, choose View | Options.
With either method, you'll see the Internet headers portion of the Message Options dialog.
In Outlook 2007, you can also click the Expand icon (see screenshot in Outlook 2010 and 2013 instructions.)
Internet Mail Issues with Microsoft Outlook
View the Message Source in Outlook
Use VBA and CDO to view or copy the headers (and source). Outlook's security warning is triggered unless you use Redemption.
Retreiving Internet Headers Using VBA in Outlook 2007/2010