-# Let exim know how the rest of the world will try to reach us.
-hosts_treat_as_local = drsnuggles.stderr.nl
+# Let exim know how the rest of the world will try to reach us. The
+# stmp transport also uses this macro for its helo command.
+EXTERNAL_HOSTNAME = drsnuggles.stderr.nl
+hosts_treat_as_local = EXTERNAL_HOSTNAME