How to remove the SMTP Service from Windows 2012 R2…
Had to install a SMTP gateway on a Windows 2012 R2 host which already had OS SMTP service installed for testing a firewall. Normally, I would have removed via Windows Features/Roles system but the option to remove feature wad greyed out. However, using PowerShell I was able to remove the service with no problems in an administratively-elevated PowerShell prompt:
Remove-WindowsFeature SMTP-Server -restart
This will restart the machine too to complete the uninstall.
I was then able to install the SMTP Gateways software as normal. This will also help on Exchange 2013 installation also.