From a560c4ac596a75492e77378b2cca349c91402e2a Mon Sep 17 00:00:00 2001
From: John Denker <jsd@av8n.com>
Date: Sun, 3 Jun 2012 22:45:45 -0700
Subject: Reject emails addressed to multiple recipients with a null envelope
 sender. A lot of your spam will be arriving with a null envelope sender. When
 those spam messages have multiple envelope recipients, they cannot be bounce
 messages.

---
 qmail-smtpd.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/qmail-smtpd.c b/qmail-smtpd.c
index 8c88c4b..23f9c89 100644
--- a/qmail-smtpd.c
+++ b/qmail-smtpd.c
@@ -58,6 +58,7 @@ void die_control() { out("421 unable to read controls (#4.3.0)\r\n"); flush(); _
 void die_ipme() { out("421 unable to figure out my IP addresses (#4.3.0)\r\n"); flush(); _exit(1); }
 void straynewline() { out("451 See http://pobox.com/~djb/docs/smtplf.html.\r\n"); flush(); _exit(1); }
 
+void err_badbounce() { out("550 sorry, bounce messages should have a single envelope recipient (#5.7.1)\r\n"); }
 void err_bmf() { out("553 sorry, your envelope sender is in my badmailfrom list (#5.7.1)\r\n"); }
 void err_nogateway() { out("553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)\r\n"); }
 void err_unimpl(arg) char *arg; { out("502 unimplemented (#5.5.1)\r\n"); }
@@ -258,6 +259,7 @@ int seenmail = 0;
 int flagbarf; /* defined if seenmail */
 stralloc mailfrom = {0};
 stralloc rcptto = {0};
+int recipcount;
 char *hostname;
 char **childargs;
 
@@ -311,6 +313,7 @@ void smtp_rcpt(arg) char *arg; {
   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");
 }
 
@@ -422,6 +425,7 @@ void smtp_data(arg) char *arg; {
 
   if (!seenmail) { err_wantmail(); return; }
   if (!rcptto.len) { err_wantrcpt(); return; }
+  if (mailfrom.len == 1 && recipcount > 1) { err_badbounce(); return; }
   seenmail = 0;
   if (databytes) bytestooverflow = databytes + 1;
   if (qmail_open(&qqt) == -1) { err_qqt(); return; }
-- 
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