Email config rediscovery - alerts via postfix only works on one server

I cannot find the setting or config or KB article that links CMK to Postfix, but somehow I got it to work in the past

Running distributed CMK Raw - one master, two secondaries using Office365 Mail connector to send from

Most of the time, alerts throw this error:
Unhandled exception: sendmail returned with exit code: 75

I need a guide that has configuration tweaks or a way to send the notifications out from the master CMK instance (my assessment was incorrect on this part)