From the change log for 2.1:
When a message is first held for approval, the notification
sent to the list admin is a 3-part multipart/mixed. The
first part holds the notification message, the second part
hold the original message, and the third part hold a cookie
confirmation message, to which the admin can respond to
approve or discard the message via email.
The consequence of this change since 2.0 is that every single junk mail
to the list (which is nearly all mail that ends up getting held for
approval) is copied in its full glory to the list administrator. This
particular list administrator experiences the further consequence of
having to give his junk mail filter mixed messages about trying to not
mark such email as junk, yet mark other email with the same contents as
junk mail. Even ignoring the filter problems, I'd just as soon not
have all that email copied to me at all. It would be sufficient to
send me an email that there is a post awaiting approval without even
providing the from address or the subject line. I can always go look
at the email in its less offensive raw text form on the Mailman
disposition web page.
So is there a way to turn this off? Is there an option for this, or
does the source code need to be modified?