[Exim] [newbie] smarthosts found by MX

Hello.

I am using Exim4.

I defined the following router for sending mail:

smart_route:
    driver = manualroute
    transport = remote_smtp
    domains = !+local_domains
    route_list = * mailhost.imag.fr

Several other smarthosts exist, but their addresses are only available
through MX records.

My question is: how can I use these MX records for sending mail,
instead of this hardcoded smarthost definition ?

Thanks in advance.

--
Nicolas