On Windows there is a very nice notification called Snarl http://snarl.fullphat.net/ which I am using since years on my machines.
I now discovered eM Client and it really looks like a nice client. As I did not find a plugins/extensions interface I am asking you to add it (otherwise I would have done in my own). There is an easy to use .NET library available at https://sites.google.com/site/snarlapp/developers/developer-guide
Best regards,
Sven