Activate same settings for Network

Hi there, I easily got this working for my main site and one of the subsites in my multisite install. However, the dashboard for all subsites that exist, there is the warning:

“Postman is not configured and is mimicking out-of-the-box WordPress email delivery.”

How can I set the defaults so that every site on my network will use the same settings? I cant go into each site and reset these settings.