. . .  consists of the parts of
an e-mail message (such as
images, stylesheets, or videos)
which are not included in
the message itself --  but are
downloaded from the Internet
each time you view the message.

• each time you view the message
• rough details about what application
  and what platform you are using
• your current geographic location
  (a rough approximation by IP address)
• that your email address is
  actually used (i.e., "active")