I should have mentioned that my method of choice and my reasoning are more applicable when the Message Pane is turned off, which is my normal layout preference. [View > Layout > Message Pane, or F8 key]

On 9/30/2010 10:02 AM, Don Tripp wrote:
[log in to unmask]" type="cite"> You can also see the headers -- without opening the email -- by viewing the Message Source. This is done by highlighting the email in question then selecting the Message Source option from the View menu (or by using the Control-U shortcut). You will see the entire email source in a new window, including all the headers.

I find this approach more convenient because you can see the full headers plus the details of any links in the body of the email (to inspect for origin). Other advantages are that it is safer (harder to click on links accidentally), doesn't trigger web bugs, wraps the headers for easier viewing (the View All Headers option lets them run off the screen to the right, making it harder to see them), and is easier to invoke because of the Control-U shortcut. YMMV.