Update metrics.conf

data/conf/rspamd/local.d/metrics.conf

@@ -33,4 +33,9 @@ group "MX" {
 	  description = "MX was ok";
 	  one_shot = "true";
 	}
+	symbol "MX_IMPLICIT" {
+	  score = -2.0;
+	  description = "MX not defined but A record at apex";
+	  one_shot = "true";
+	}
 }