SmarterMail 16.x and 17.x share the same system requirements. Both of these versions may work just fine on a Windows 2008 R2. However, we highly recommend Windows Server 2012 R2 or higher for two main reasons:
The end-of-life mainstream support for Windows Server 2008 R2 ended back on January 13, 2015, and on January 14, 2020, Microsoft will end all support for Windows Server 2008 R2.
More importantly, the lack of web sockets in IIS 7.0/7.5 prevent SmarterMail from utilizing SignalR, which is what allows the server to send real-time communication to client-side web applications. Without it, your users may experience delays within the web interface since they would be leveraging Long Polling rather than SignalR. Users may also experience issues with items refreshing within their web interface, chat messages coming through in a delayed fashion, Inbox failing to update, notes not refreshing upon save, etc.
Long polling will be the fallback solution if web sockets are not in use, but the best experience would be on Windows 2012 R2 or higher.
I hope this helps!