from: http://websteci.wordpress.com/2012/12/12/installing-tftp-and-smtp-servers-in-windows-server-2012/
Steps to install SMTP Server in Windows Server 2012:
Click the Server Manager button on the bottom left of the Windows task bar
On the top right menu of Server Manager, click Manage and then Add Roles and Features, which opens the Add Roles and Features Wizard.
Read the initial information page in the Add Roles and Features Wizard and click Next.
On the Installation Type page select “Role-based or feature-based installation” then click Next.
On the Server Selection page select “Select a server from the server pool”, then select your server from the list (there should be only one), then click next.
On the Server Roles page, go ahead and click Next, there is no need to select anything here at this point.
On the Select Features page, scroll down and select SMTP Server and TFTP Client then click Next.
On the Confirmation page look in the right box to make sure what you selected is there, then click Install at the bottom of the page (when installed you may have to restart).