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
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
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