[exim] route_list / remote smtp defer

Hi,

I contact you because I've got some troubles in configuring route_list
option in exim4.conf.
First, here is what I want to do :
I have a first e-mail server in a public DMZ. This e-mail server receive
e-mails and transmit local e-mails to a second e-mail server.

e-mail from the web -> first server in a DMZ : "PUBLICSERVER" -> Second
server behind another firewall "PRIVATESERVER"

I have configured route-list option and it works great. But when I want to
send e-mails from PRIVATESERVER to PUBLICSERVER it fails : I'got a "remote
smtp defer" error message in the log file.
But from the PRIVATESERVER I can telnet the PUBLICSERVER on 25th port.

Maybe I have done something wrong ?

 smart_route:
    driver = manualroute
    domains = !+local_domains
    transport = remote_smtp
    route_list = * 192.168.101.10

thanks for your precious help ;-)
JPB