[Exim] dnsdb query

hi,

I am secondary MX for 2 domain.
I store there mail in bsmtp format and then they get to there final host by
another mean.

when i send a mail from my host runing exim to thoses domain,
i would like the mail to first try to be sent to the primary MX, and if fail
store it in the bsmtp file.

Thanks to Nico Erfurth for the help about how to try only once.

I am trying not to have the primary MX name coded in a file when routing the
first time for thoses domain.
So i would like to do a dnsdb query extracting the list of the primary MX host
to feed the route_list option.

how can i do that ?

thank you for your help

Edouard