diff --unidirectional-new-file -ur qmail-1.03.orig/qmail-smtpd.c qmail-1.03/qmail-smtpd.c
--- qmail-1.03.orig/qmail-smtpd.c	2005-01-06 01:22:13.841316160 +0100
+++ qmail-1.03/qmail-smtpd.c	2005-01-06 01:25:34.862756256 +0100
@@ -132,6 +132,7 @@
   out(" (#5.7.1)\r\n");
 }
 #endif
+void err_badbounce() { out("550 sorry, bounce messages should have a single envelope recipient (#5.7.1)\r\n"); }
 void err_unimpl() { out("502 unimplemented (#5.5.1)\r\n"); }
 void err_syntax() { out("555 syntax error (#5.5.4)\r\n"); }
 void err_relay() { out("553 we don't relay (#5.7.1)\r\n"); }
@@ -429,6 +430,7 @@
 stralloc rcptto = {0};
 stralloc fuser = {0};
 stralloc mfparms = {0};
+int recipcount;
 
 int mailfrom_size(arg) char *arg;
 {
@@ -541,6 +543,7 @@
   if (!stralloc_copys(&rcptto,"")) die_nomem();
   if (!stralloc_copys(&mailfrom,addr.s)) die_nomem();
   if (!stralloc_0(&mailfrom)) die_nomem();
+  recipcount = 0;
   out("250 ok\r\n");
 }
 void smtp_rcpt(arg) char *arg; {
@@ -580,6 +583,7 @@
   if (!stralloc_cats(&rcptto,"T")) die_nomem();
   if (!stralloc_cats(&rcptto,addr.s)) die_nomem();
   if (!stralloc_0(&rcptto)) die_nomem();
+  recipcount++;
   out("250 ok\r\n");
 }
 
@@ -696,6 +700,7 @@
  
   if (!seenmail) { err_wantmail(); return; }
   if (!rcptto.len) { err_wantrcpt(); return; }
+  if (mailfrom.len == 1 && recipcount > 1) { err_badbounce(); return; }
   if (flagbrt) { err_brt(); return; }
   seenmail = 0;
   if (databytes) bytestooverflow = databytes + 1;