[Exim] exim and routing to private internal network.

Hi,

I was wondering what one needs to do to make a public MTA, running exim
4.10, route various domains to an internal "private" group of servers
running the same version of exim.

Something like this

localdomain ---> Main MTA ---> local delivery * currently works well.
otherdomain ---> Main MTA ---> some other internal MTA

I don't if this makes any sense, but if it can work like the above
"diagram" that would be great.

Thanks

--bash