Thanks for reaching out! The main focus for SmarterMail 16.x has been on the user interface redesign, ease of administration, introduction of web conferencing -- like screen sharing and video chat -- and exposing all interface functionality to the API. At this moment, we are not developing a plugin or email client alternative; however, by exposing all functionality to the API, we'll have unlocked endless possibilities for companies or administrators to create their own tools / clients or expand the reach of their system with new integrations.
That said, we have made improvements to SmarterMail's syncing functionality with substantial changes to the EWS protocol. EAS also saw some updates, and improvements for both of these protocols should be pushed to version 15.x as well.
I hope this helps!