Ask HN: Cross platform alternatives to Google Inbox client?

While migrating away from gmail servers is one matter, the main thing keeping me on Google's service is Inbox. It's quite simply revolutionised the way I deal with my mail, and I can't really come to terms with using a traditional mail client again.

If I were to switch to some IMAP-providing alternative to gmail servers, is there a frontend product I can use that gives me an Inbox-like experience on the frontend? My requirements are:

* Bundling of newsletters with no notifications, pinning * Snoozing * Undo send (delayed sending) * Good search * Cross platform

Nice to have:

* Open source * Notification sync * Attachment preview