The template builder software is not a responsive design, however it will work OK on a mobile device, but depending on the exact device will depend on how the email is displayed.
If you use an email responsive design within our e-Marketing platform - for example if you have HTML code to paste into the template then the system is able to deal with these responsive designs by storing CSS / HTML / etc so that the email will be displayed correctly on a mobile device.
If you have any questions please contact our Support Helpdesk.