I'd love to know this too if someone has any good suggestions.
I recently switched over from Outlook (which I found extremely annoying) to eM Client (not open-source unfortunately but free for 2 inboxes) which I like leagues better than Outlook. The only real open-source Outlook alternative (I know of) would be Thunderbird, if you haven't found it already. Haven't used it in a long while though. For Android I am using k9-Mail which is open-source but still pretty barebones. It's from the same guys as Thunderbird, or rather k9 Mail was taken over by the Thunderbird guys. For my needs it's perfectly fine but it can't do anything besides mail.