Here is a beta email app that sort of combines Gmail and Apple Mail, I think. I've not used it, and even after reading the info on the website I'm not clear of what advantages it offers. I think I'll have to read it again. I do use Gmail, and not Apple Mail, so I've not decided whether to try this app or not. If, however, you try Mimestream, let me know, if you would, what you think of it.
Jim Hamm