Q:

Is there an Akonadi-enabled message notifier in Kubuntu (which does not require KMail running)? 

IMHO one of the cool things about KDE's groupware cache Akonadi is that it synchronises and indexes your mail even if there is no e-mail software running.  I always thought that one of the prime motivations for this is a message notifier that does not require a running mail application.  But there is no such thing! 
Or am I simply too stupid to find the settings?  Things I am aware of:

KMail's native notifications, which only work with KMail running. 
Plasmoids of IMHO somewhat poor quality, which do not work with Akonadi and do not integrate as nicely into KDE's notification system. 
A set of plasmoids dubbed Lion Mail developed by Sebastian Krüger  would do the job.  However, I found neither packages nor source on the web and even KDE PIM developers apparently don't have a clue about its status, so I assume it is dead. 

So: Is there a mail notifier which only requires Akonadi and integrates nicely into the Plasma workspace?  Please, I'm desperate! :) 

A:

I now at least figured out how to find and install LionMail under Kubuntu 11.10.  I'm not perfectly happy with it since it does not integrate into the message indicator framework, but it works very well together with Akonadi. 

Prerequisites: For compiling you need at least these packages (I hope I did not forget some):
sudo apt-get install cmake kdelibs5-dev kdepimlibs5-dev libakonadi-dev libqtwebkit-dev

Source code: There is a (somewhat outdated) source tarball.  Alternatively, get the latest source code from the git repository:
git clone git://anongit.kde.org/plasma-lionmail

Installation: Change into the source directory, build and install the widget
cd plasma-lionmail
cmake .  -DCMAKE_INSTALL_PREFIX=/usr
make
sudo make install

If you only want to install the widget for yourself, use ~/.local instead of /usr as a prefix.  You do not need to sudo the installation in this case. 
Add widgets: Unlock your panel and add the widget 'E-mail notifier'.  Viola! 

I will still leave the question open if somebody comes up with a better widget or solution (if anyone is still interested).