https://markup.allizom.org/en-US/newsletter/ has RESPONSYS_ID = "ResponsysIdHere" This should be updated to our current production code for testing. Please do not post the value in this bug.
The value is in https://bugzilla.mozilla.org/show_bug.cgi?id=646640#c5
I've tweaked settings_local.py.default to match our other apps. RESPONSYS = "ResponsysIdHere" should be the secret code.