email: Fixed up exim config for forwarding mail to

"non-local-but-local" domains from non-local domains.  Ticket #620219

email/exim4/symbiosis.d/20-routers/21-vhost-no-local-mail

 # Like the last router, this tries DNS lookups for vhost domains with no
 # mailbox directory, and no default forwarding rule set up.
+#
+# This router will get run if any of
+#
+#  the message has a message ID already (i.e. it is already in the queue), or
+#  the sender_host_address is empty (i.e. the message was accepted locally), or
+#  the sender_host_address is allowed to relay, or
+#  the message was sent by an authenticated user,
+#
+# are true, AND none of the following files exist
+#
+#  /srv/$domain/mailboxes
+#  /srv/$domain/config/default_forward
+#  /srv/$domain/config/aliases
+#
+#
 
 vhost_no_local_mail:
   debug_print = "R: vhost_no_local_mail for $local_part@$domain"
@@ -7,6 +22,7 @@ vhost_no_local_mail:
   domains = +vhost_domains
   condition  = ${if and{ \
                  {or { \
+                   {!eq{$message_exim_id}{}} \
                    {eq{$sender_host_address}{}} \
                    {match_ip{$sender_host_address}{+relay_from_hosts}} \
                    {!eq{$authenticated_id}{}} \