The least worst choice, for me, is Gmail on a web browser. Switched over from Thunderbird and prior to that Evolution after many years of using both.

Don't misunderstand. I do NOT want to use Gmail, and there are a lot of things about it that fall short... but nothing else has given me such pain-free integration with calendar, contact, and office apps, on my desktop, my laptop, and my phone.

I'm also keeping an eye on ownCloud, which seems to be on its way to become a viable alternative.