diff -ruN netqmail-1.06-original/chkuser.c netqmail-1.06/chkuser.c
--- netqmail-1.06-original/chkuser.c	2021-09-27 19:22:33.665033761 +0200
+++ netqmail-1.06/chkuser.c	2021-09-24 21:12:42.559628632 +0200
@@ -327,9 +327,11 @@
 	if (strncmp (domain->s, "xn--", 4) == 0) {
 		if (strstr (&domain->s[4], "--") != NULL)
 			return 0;
+/* allowing domains with hyphens like y--s.co.jp
 	} else {
 		if (strstr (domain->s, "--") != NULL)
 			return 0;
+*/
 	}
         if (strstr (domain->s, ".-") != NULL) {
                 return 0;