"LISTSERV list owners' forum" <[log in to unmask]> wrote on 
03/30/2005 07:35:29 PM:

> (If you choose Thunderbird you only need to open the e-mail, click 
CTRL+U
> and it'll be shown in all its raw glory: full headers, MIME boundaries,
> binary encoding....Bewwwdiful!!)

Even better!!  In Thunderbird, you do not even have to open the message. 
From the inbox, highlight the message and just hit Ctrl-U to see the 
message source. 

This is especially usefull for evaluating a suspect message without 
opening it and triggering any web tracking URLs, etc.