I've been looking for an initiative like this for so many years! Not even a month ago, I spent an afternoon looking all over the internet for somebody trying to build a native mail client for MacOs. Since DéjàVu from the lead dev of Sparrow, nobody it seems nobody has tried to do this. I haven't yet tested Mimestream but I already praise you for trying to do it! I will definitely give it a try and pay for it!