Version:           1.6.2 (using KDE 3.2.2,  (testing/unstable))
Compiler:          gcc version 3.3.3 (Debian 20040401)
OS:                Linux (i686) release 2.6.5

"New mail arrived" isn't very usefull

How about have "
New Message for <account name>
Title: <title>
From:  <from>"

and maybe show the first 3 or so lines of the plain text part of the message.

If there is more than one message ... then cycle thru the list of new messages.  If the "fade" setting is on in styles then maybe use a fade between each message preview.

When the user clicks on the popup it should take the user straight to that message in kmail.

And maybe have a back forward button to go back to the previous passive popup, but that maybe overkill.