[microtonal]

They are excellent! I had a feature request twice, it was implemented on their beta server within days. Their (standard-conforming) IMAP and Webmail is really fast (they cache recent stuff on SSDs). They have apps now for Android and iOS (basically wrappers around the web app with notification support, but they work well enough). They recently added CalDAV and CardDAV support.

On top of all of that, they actively contribute to open source projects such as Cyrus.