use $listname-bounces not /dev/null (aka postmaster) as the sender.
some mail systems, including upas, reject email to multiple recipients
from /dev/null.

Reference: /n/atom/patch/applied2013/ml-bounces
Date: Thu Jul 11 18:15:07 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/ml/common.c	Thu Jul 11 18:14:12 2013
+++ /sys/src/cmd/upas/ml/common.c	Thu Jul 11 18:14:12 2013
@@ -112,16 +112,25 @@
 	Bterm(b);
 }
 
+static void
+setsender(char *name)
+{
+	char *s;
+
+	s = smprint("%s-bounces", name);
+	putenv("upasname", s);
+	free(s);
+}
+
 /* start a mailer sending to all the receivers */
 int
 startmailer(char *name)
 {
-	int pfd[2];
 	char **av;
-	int ac;
+	int pfd[2], ac;
 	Addr *a;
 
-	putenv("upasname", "/dev/null");
+	setsender(name);
 	if(pipe(pfd) < 0)
 		sysfatal("creating pipe: %r");
 	switch(fork()){
@@ -159,7 +168,7 @@
 	int pfd[2];
 	Waitmsg *w;
 
-	putenv("upasname", "/dev/null");
+	setsender(listname);
 	if(pipe(pfd) < 0)
 		sysfatal("creating pipe: %r");
 	switch(fork()){