Dear Felix,
i tried another mail provider and got it working with web.de 
I first tried gmail but ran into security issues. But more importantly it seems to be a provider relatet problem… I’m confused as it worked in may also with yahoo. Maybe yahoo made some changes?
anyway… i would prefer to use the yahoo adress as i set it up just for this case. if you have any idea how to go on with this issue, i would likely test further things.