Thanks to John Cummings.

src/cmd/upas/send/tryit

@@ -0,0 +1,29 @@
+#!/bin/sh
+set -x
+
+> /usr/spool/mail/test.local
+echo "Forward to test.local" > /usr/spool/mail/test.forward
+echo "Pipe to cat > /tmp/test.mail" > /usr/spool/mail/test.pipe
+chmod 644 /usr/spool/mail/test.pipe
+
+mail test.local <<EOF
+mailed to test.local
+EOF
+mail test.forward <<EOF
+mailed to test.forward
+EOF
+mail test.pipe <<EOF
+mailed to test.pipe
+EOF
+mail dutoit!bowell!test.local <<EOF
+mailed to dutoit!bowell!test.local
+EOF
+
+sleep 60
+
+ls -l /usr/spool/mail/test.*
+ls -l /tmp/test.mail
+echo ">>>test.local<<<"
+cat /usr/spool/mail/test.local
+echo ">>>test.mail<<<"
+cat /tmp/test.mail