add DNS checker and run against suspicious hosts

external/cmd/signup/main.go

@@ -137,8 +137,7 @@ func DigMX(raw string) (domains []string, err error) {
 		records, e := net.LookupMX(host[1:])
 		if e != nil {
 			err = ErrNoSuchDomain
-		}
+		} else if len(records) == 0 {
-		if len(records) == 0 {
 			err = ErrNoSuchMailserver
 		} else {
 			ok = true