Thanks to John Cummings.

src/cmd/upas/send/gateway.c

@@ -0,0 +1,24 @@
+#include "common.h"
+#include "send.h"
+
+#undef isspace
+#define isspace(c) ((c)==' ' || (c)=='\t' || (c)=='\n')
+
+/*
+ *  Translate the last component of the sender address.  If the translation
+ *  yields the same address, replace the sender with its last component.
+ */
+extern void
+gateway(message *mp)
+{
+	char *base;
+	String *s;
+
+	/* first remove all systems equivalent to us */
+	base = skipequiv(s_to_c(mp->sender));
+	if(base != s_to_c(mp->sender)){
+		s = mp->sender;
+		mp->sender = s_copy(base);
+		s_free(s);
+	}
+}