notifications do not get send out

Hi there

I enabled notifications and messaging (6.x-2.2) and the simple Mail Method for messaging.
I can see that notifications get in the queue, but they don't get send out.
Also runnign the process manually (not waiting forn cron) doesn't help.

the drupal internal contact form works totally fine (it's using the same mail method, right?)

I tried clearing the cache, etc. (including reboot) --> nothing, it just hangs oin the queue

can anyone point me in the right direction? Maybe a guide on how I can test the modules seperately?

Thanks
Andre