My email client of choice is Apple Mail. Thankfully, it is very easy to to view email headers in Apple Mail...
1. Open Apple Mail.
2. Double-click to open the email message.
3. Choose View at top menu and select Customize Toolbars.
4. Drag the Full Headers or Long Headers icon into your toolbar and save changes.
or
1. Select your message.
2. Click View -> Message -> Long Headers
