feat: add the ability to set a MAILFROM value for local cron emails

Closes #7419

plugins/cron/set.go

@@ -3,6 +3,10 @@ package cron
 import "errors"
 
 func validateSetValue(appName string, key string, value string) error {
+	if key == "mailfrom" && appName != "--global" {
+		return errors.New("Property cannot be specified on a per-app basis")
+	}
+
 	if key == "mailto" && appName != "--global" {
 		return errors.New("Property cannot be specified on a per-app basis")
 	}