credit to jas for pointing out the issue.
cat >> mailbox is no longer sufficient, as we
might be using mdir(6) format.

Reference: /n/atom/patch/applied2013/sendappend
Date: Wed Dec 11 14:56:42 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/send/cat_mail.c	Wed Dec 11 14:56:08 2013
+++ /sys/src/cmd/upas/send/cat_mail.c	Wed Dec 11 14:56:09 2013
@@ -88,7 +88,7 @@
 	s = s_to_c(ss);
 	if (flagn) {
 		if(!flagx)
-			print("cat >> %s\n", s);
+			print("upas/mbappend %s\n", s);
 		else
 			print("%s\n", s_to_c(dp->addr));
 		s_free(ss);