They contain such boring information as which server picked the message up from which other server at which time. While not particularly interesting normally, this information is required to identify the true origin of an email message, in particular of spam.
Like many other options, the ability to show these headers does exist in Outlook, but it is a bit hidden.
View All Message Headers in Outlook
To have Outlook 2007 show you a message's all header lines:
- Open the email in a new window by double-clicking it.
- Click the expansion button in the lower right corner of the Tags toolbar box.
- The box, by default, holds the Follow Up and Mark as Unread buttons.
- In Outlook 2007, the box is labeled Options.
- Find the headers under Internet headers:.
View All Message Headers in Outlook 2000, 2002 and 2003
To display all of a message's headers lines in Outlook 2000 to Outlook 2003:
- Open the message in a new window in Outlook.
- Select View | Options... from the message's menu.
All header lines appear under Internet Headers at the bottom of the dialog that comes up.
See the Complete Source (Headers and Message Body)
With a little tweaking of the Windows registry, you can also make Outlook display the complete, original and unedited message source.
(Updated June 2012)