Microsoft announced today a preview build of Exchange Server 2019. You can download it here.
“We strongly believe Office 365 delivers the best and most cost-effective experience to our customers, but we understand that some customers have reasons to remain on-premises. Exchange Server 2019 is designed to deliver security, performance, and improved administration and management capabilities. These are the attributes our largest on-premises customers tell us they need from Exchange. We also have features end-users will love too of course.”
Read more at the EHLO blog.