Yep, I already annoyed several people in my forum with test messages.

I still have
die("test"); as plugin code, but it just takes me to my inbox again after sending.
Also tried to hook
private_insertpm_process, which should be triggered both, when actually sending the pm and when just displaying a preview... But no luck.
Its either me being very stupid or something is really broken... (Running 3.6.7 btw)
EDIT: Hooking
private_start works...