I would look at the support for Exchange 2007 as a solid indicator as that’s what you are probably migrating your email off of.
For Exchange 2007, the last support date is April 11, 2017.
Finding the same information for Windows server 2008 is nearly horrific. I had to look at Vista sp2 as the matching desktop OS to get the support date.
|Windows Vista Service Pack 2||4/29/2009||4/10/2012||4/11/2017|
|Microsoft Exchange Server 2007 Service Pack 3||6/7/2010||4/10/2012||4/11/2017|