Falls short of being a worthwhile competitor to Apple Mail, Spark, and Edison. HTML doesn't always display correctly and trying to set this as displayable in settings doesn't always work. It also has an extraordinarily poor spam filter (although I didn't bother trying to find a solution to this, as Spark and Edison do a pretty good job on their own). Apple mailbox folders must be subscribed to within a special not-easy-to-find window or it will appear as if your folders aren't being synchronized (the only negative when it comes to the interface). The calendar is very limited in capabilities, it won't directly interface with Apple Calendar due to an inability to read the Apple format. Why this hasn't been fixed I don't know. There is a workaround but it involves publishing your iCloud Calendar on the iCloud website as "Public" and then subscribing to it on the MS 365 website portal - which raises security issues. The type on the calendar is also small and not scalable, and events will not match colors selected in Apple Calendar even if the workaround is used. This is a real shame since it has a great layout (I miss Entourage!) and has a nice, easy-to-navigate GUI.