The interface is very well organized. Setup is easy. Tracking and activity feeds are very useful.
Often when forwarding emails, the server based attachments, even small <100kb signature files fail to attach to the new message effectively disabling the ability to send.
There is also a common occurrence of the program relying so much on continuous server syncing that it sometimes can't keep up with real time user input which results in accidental keystroke recognition sending emails to archive or trash or draft mode.
Commonly emails get stuck in draft mode.
Periodically check your draft folder for unsent emails.
I simply enjoy the better designed interface in comparison to GMail as well as tracking activity.