If you wish to stop email service from Smarter Mail V12 for any particular domain, then you can follow the below mentioned steps:

[1] Login to your Smarter Mail administrator account. Once you login search the domain for which you wish to disable email service:

[2] Now right click on domain name and go to 'options' tab. Tick mark the check box infront of 'Disable Domain' and click on 'Save' button.