While Spark is very appealing aesthetically, I would not recommend using if you wish to keep your email contents private.
Modern & feature rich
Major privacy concerns
Tech support responses can take weeks (if they respond at all) and they offer no fix.
Calendar does not notify you of events (icloud) -
Some emails come in and are marked as "read" so you miss them
I would love to be able to send multiple meeting options to clients. Like Boomerang for Gmail's option.
Cleaning up your inbox
As a mail client, none.
The besp mailing app i`ve found so far!.
Interface and usabiloity are awesome.
With one swipe you can manage all your inbox