[Exim] Exclude Null sender from router

Hello the list,

I have a little problem whis my configuration : I set up a router to use
a smarthost for some domains (aol, hotmail). So far, no problem until I
experienced a typo in the file search for the relay domain. I then got a
message from mailer-daemon notfying the typo. But the problem is that
the error message fails to deliver as the null sender can't use the
smarthost router, as this one expects a valid home dir.

I tried to add
- a condition to test the null sender envelope
- deny the router for null sender

No luck.

Can someone help me?

Thanks.

Here is the smarthost router.

exterieur_perso:

driver = manualroute
senders = ! : ! ^\$ <= doesn't work
transport = remote_smtp
require_files = ${sender_address_local_part}:/home/${sender_address_local_part}/exim4/relais
route_data = ${lookup{$domain}lsearch{/home/${sender_address_local_part}/exim4/relais}}