Step 1: Launch the Server Manager. From the Search charm, type in Server Manager to find it.
Step 2: From the Dashboard, Add Roles and Features. The Add Roles and Features Wizard will begin. Click Next on the first screen.
Step 3: Select Role-based or feature-based installation.
Step 4: Select the Essentials Server (which should be highlighted by default).
Step 5: Click Next to bypass the Roles selections.
Step 6: Scroll down the list and tick the SMTP Server feature.
Step 7: A new window will pop up to inform you that some other services will also be installed. Click Add Features to confirm and continue. Click Next to continue past the features selection screen.
Step 8: Click Install to complete the Installation.
Step 9: Click Close when the installation has completed.