https://tutanota.com/blog/posts/green-email/
Any news on that question?
("from whom (which power company ) do you / your datacenter buy that ' green energy' ?" )
OK, thanks a lot for the answer, though it was not was I was hoping for! :-(
As far as I can tell #innogy is owned either by #EON or by #RWE (they have been - and I think still are - selling it back and forth, probably to confuse people and for #greenwashing), and both are NOT allies on the path towards #greenenergy.
I wouldn't want for any of them to get my money! :-(
Did you consider other options or will you do so in the future?

I'm still waiting on Tuta to begin using LESS energy, the apps are hardly carbon friendly. They are the heaviest email apps I've used, and still offer a significantly poorer experience on older hardware. While choice of energy providers might be somewhat out of your control, the efficiency of your code is not, and the collective energy draw by your users running your applications, I imagine, is sizable. +1 to cutting client side energy use as well.