[Exim] distributed mail spool

Hello,

how can I setup a distributed mail spool? I have several webservers,
that are sending their messages directly to the recipient. But instead,
I want them to react as a smart host, so that they just deliver the
messages to a mail-out spool where the final delivery is done. The smart
host thing is not the problem. But if too much webservers share one
mail-out spool, it takes too much time until the messages are delivered.
So my idea is that the webservers are sending their messages to one of
the mail-out spools round robin or randomized. How could I do this? Is
there any possiblity to get a random entry of a list in an expansion?

Regards
Marten